When a store is propagated to all stores succeeding it in the same basic
block, and we encounter another store, we can delete the previous one.

This optimizes the StoresInMultipleBlocks test's generated code from:

 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

Into:

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

Bug: b/179668593
Change-Id: I2036d7b7c97be17ce73ff32234fe51841409a20c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52568
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>