cmd/compile/internal/ssa: tighten loads

This change introduces alias analysis into the ssa
backend, and uses it to tighten some loads with their uses.

Given code like

    res, ok := fn()
    if !ok {
        return
    }
    // use res

The 'res' return value won't be loaded until after examining 'ok.'

Alias analysis also enables tightening of loads from
constant memory (strings) and certain autotmp stack slots.

A follow-up CL will introduce alias analysis to dead-store
elimination. Another will introduce load elimination.

Fixed golang#19195

Benchmarks on linux/arm:
name                     old time/op    new time/op     delta
BinaryTree17-4              31.1s ± 0%      30.9s ± 0%   -0.39%  (p=0.001 n=9+8)
Fannkuch11-4                13.9s ± 0%      14.1s ± 0%   +0.90%  (p=0.000 n=8+9)
FmtFprintfEmpty-4           666ns ± 5%      674ns ± 4%     ~     (p=0.424 n=10+10)
FmtFprintfString-4         1.09µs ± 4%     1.12µs ± 4%   +2.62%  (p=0.027 n=10+10)
FmtFprintfInt-4            1.22µs ± 3%     1.16µs ± 3%   -4.37%  (p=0.000 n=10+10)
FmtFprintfIntInt-4         1.74µs ± 2%     1.64µs ± 3%   -6.00%  (p=0.000 n=10+10)
FmtFprintfPrefixedInt-4    1.75µs ± 1%     1.69µs ± 3%   -3.80%  (p=0.000 n=9+10)
FmtFprintfFloat-4          3.27µs ± 1%     3.41µs ± 1%   +4.33%  (p=0.000 n=10+9)
FmtManyArgs-4              6.49µs ± 1%     6.23µs ± 2%   -4.10%  (p=0.000 n=10+10)
GobDecode-4                87.1ms ± 1%     75.5ms ± 1%  -13.37%  (p=0.000 n=10+10)
GobEncode-4                69.4ms ± 1%     69.7ms ± 1%     ~     (p=0.190 n=10+10)
Gzip-4                      3.56s ± 1%      3.57s ± 1%     ~     (p=0.053 n=10+9)
Gunzip-4                    446ms ± 2%      442ms ± 2%     ~     (p=0.123 n=10+10)
HTTPClientServer-4         1.51ms ± 1%     1.55ms ± 3%   +2.26%  (p=0.001 n=8+9)
JSONEncode-4                191ms ± 1%      175ms ± 2%   -8.33%  (p=0.000 n=10+10)
JSONDecode-4                798ms ± 1%      835ms ± 1%   +4.65%  (p=0.000 n=10+10)
Mandelbrot200-4            33.6ms ± 0%     33.6ms ± 0%     ~     (p=0.068 n=8+10)
GoParse-4                  42.4ms ± 1%     42.5ms ± 1%     ~     (p=0.190 n=10+10)
RegexpMatchEasy0_32-4       829ns ± 1%      853ns ± 1%   +2.98%  (p=0.000 n=9+8)
RegexpMatchEasy0_1K-4      4.04µs ± 1%     4.03µs ± 1%     ~     (p=0.986 n=10+10)
RegexpMatchEasy1_32-4       889ns ± 2%      900ns ± 5%     ~     (p=0.566 n=10+10)
RegexpMatchEasy1_1K-4      6.01µs ± 2%     6.15µs ± 2%   +2.29%  (p=0.000 n=9+9)
RegexpMatchMedium_32-4     1.35µs ± 3%     1.39µs ± 4%   +2.26%  (p=0.018 n=9+10)
RegexpMatchMedium_1K-4      357µs ± 9%      352µs ± 2%     ~     (p=0.968 n=10+9)
RegexpMatchHard_32-4       22.2µs ± 6%     22.6µs ± 6%     ~     (p=0.161 n=9+9)
RegexpMatchHard_1K-4        652µs ± 4%      664µs ± 4%   +1.91%  (p=0.028 n=9+10)
Revcomp-4                  51.4ms ± 1%     51.3ms ± 2%     ~     (p=0.353 n=10+10)
Template-4                  1.17s ± 2%      1.06s ± 2%   -9.39%  (p=0.000 n=10+10)
TimeParse-4                4.44µs ± 1%     4.46µs ± 1%   +0.50%  (p=0.003 n=9+10)
TimeFormat-4               9.30µs ± 1%     9.33µs ± 1%     ~     (p=0.197 n=10+10)
[Geo mean]                  557µs           553µs        -0.82%

Change-Id: Ibe35216e1bc3b0ff937cee9b3bac64f40f087b61