Missed optimization: Option<fieldless enum> equality

[Vlad-Shcherbina]

This is similar to #49892, but different.
It's not limited to non-zero integer types. This happens for any types that have holes but no uninitialized bits.
I think comparing Option<some fieldless enum> values is not uncommon, so it adds to the severity of the issue.

To reproduce

1. Create the following program:

   pub fn eq(x: Option<bool>, y: Option<bool>) -> bool {
       x == y
   }

2. Inspect generated code, for example by running

   cargo rustc --release --lib -- --emit asm -Cllvm-args=--x86-asm-syntax=intel
   

   Or just visit godbolt.

Expected result

eq:
        cmp     dil, sil
        sete    al
        ret

Actual result

eq:
        cmp     dil, 2
        sete    al
        cmp     sil, 2
        setne   cl
        cmp     al, cl
        je      .LBB0_1
        mov     al, 1
        cmp     dil, 2
        je      .LBB0_5
        cmp     sil, 2
        je      .LBB0_5
        test    dil, dil
        sete    cl
        test    sil, sil
        setne   al
        xor     al, cl
.LBB0_5:
        ret
.LBB0_1:
        xor     eax, eax
        ret

Meta

This happens both on 1.43.0 stable and on the recent nightly:

> rustc --version --verbose
rustc 1.45.0-nightly (8970e8bcf 2020-05-23)
binary: rustc
commit-hash: 8970e8bcf6153d1ead2283f1a0ed7b192230eca6
commit-date: 2020-05-23
host: x86_64-pc-windows-msvc
release: 1.45.0-nightly
LLVM version: 10.0

[veera-sivarajan]

This is same as #122734

https://rust.godbolt.org/z/MbvvK8e59

@rustbot label +a-llvm

[veera-sivarajan]

@rustbot label llvm-fixed-upstream