For loads following stores in the same basic block, replace their result
with the data that was stored.

It transforms the output of the StoresInMultipleBlocks test from:

 sub         rsp,38h
 mov         dword ptr [rsp],0Dh
 cmp         ecx,0
 je          a
 mov         dword ptr [rsp],4
 add         dword ptr [rsp],3
b:
 mov         eax,dword ptr [rsp]
 add         rsp,38h
 ret
a:
 mov         dword ptr [rsp],6
 add         dword ptr [rsp],5
 jmp         b

Into:

 sub         rsp,38h
 mov         dword ptr [rsp],0Dh
 cmp         ecx,0
 je          a
 mov         dword ptr [rsp],4
 mov         eax,4
 add         eax,3
 mov         dword ptr [rsp],eax
b:
 mov         eax,dword ptr [rsp]
 add         rsp,38h
 ret
a:
 mov         dword ptr [rsp],6
 mov         eax,6
 add         eax,5
 mov         dword ptr [rsp],eax
 jmp         b

While at first this might seem like a regression, note that
`add [rsp],3` performs both a load and a store. The optimization pass
eliminated two load operations from this test. The redundant stores will
be eliminated by a subsequent change.

Also adds a unit test for the case where store-to-load propagation
should not be performed due to an indirect store through a pointer
happening in between.

Bug: b/179668593
Change-Id: I6ca133ac4e77bbbc3efd517dff6e8dee6d2dc147
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52533
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>