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Instructions with immediates can be precomputed when operating on a
constant - do so for SLTI/SLTIU, SLLI/SRLI/SRAI, NEG/NEGW, ANDI, ORI
and ADDI. Additionally, optimise ANDI and ORI when the immediate is
all ones or all zeroes.

In particular, the RISCV64 logical left and right shift rules
(Lsh*x*/Rsh*Ux*) produce sequences that check if the shift amount
exceeds 64 and if so returns zero. When the shift amount is a
constant we can precompute and eliminate the filter entirely.

Likewise the arithmetic right shift rules produce sequences that
check if the shift amount exceeds 64 and if so, ensures that the
lower six bits of the shift are all ones. When the shift amount
is a constant we can precompute the shift value.

Arithmetic right shift sequences like:

   117fc:       00100513                li      a0,1
   11800:       04053593                sltiu   a1,a0,64
   11804:       fff5859                addi    a1,a1,-1
   11808:       0015e593                ori     a1,a1,1
   1180c:       40b45433                sra     s0,s0,a1

Are now a single srai instruction:

   117fc:       40145413                srai    s0,s0,0x1

Likewise for logical left shift (and logical right shift):

   1d560:       01100413                li      s0,17
   1d564:       04043413                sltiu   s0,s0,64
   1d568:       40800433                neg     s0,s0
   1d56c:       01131493                slli    s1,t1,0x11
   1d570:       0084f433                and     s0,s1,s0

Which are now a single slli (or srli) instruction:

   1d120:       01131413                slli    s0,t1,0x11

This removes more than 30,000 instructions from the Go binary and
should improve performance in a variety of areas - of note
runtime.makemap_small drops from 48 to 36 instructions. Similar
gains exist in at least other parts of runtime and math/bits.

Change-Id: I33f6f3d1fd36d9ff1bda706997162bfe4bb859b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/350689
Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Michael Munday <mike.munday@lowrisc.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
fe8347b

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