Pointers to the base address of arrays often get stored and then loaded
multiple time. This change adds an optimization pass to replace these
loads with the result of the alloca of the array, and eliminates the
allocas of the pointer variables.

The PropagateAlloca unit test without this optimization produces:

 sub         rsp,38h
 mov         dword ptr [rsp+8],16h
 cmp         ecx,0
 je          000002543C4E407E
 lea         rax,[rsp+8]
 mov         qword ptr [rsp],rax
 mov         rax,qword ptr [rsp]
 mov         eax,dword ptr [rax]
 add         rsp,38h
 ret

With the optimization pass it becomes:

 sub         rsp,38h
 mov         dword ptr [rsp],16h
 cmp         ecx,0
 jne         0000015DC3B33074
 mov         eax,dword ptr [rsp]
 add         rsp,38h
 ret

Also add a couple of unit tests for corner cases where the propagation
is not safe to perform.

Bug: b/179279298
Change-Id: I784899319bf5360f47c6fbc22fb82134d135dbfc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52468
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>