This change implements the Scalar Replacement of Aggregates (SRoA)
optimization. Since Reactor only supports array aggregates, this
replaces arrays which are only indexed by static indices with individual
variables for each element.

The ReactorArray test is optimized from:

 sub         rsp,38h
 mov         dword ptr [rsp],1
 mov         dword ptr [rsp+4],2
 mov         eax,dword ptr [rsp]
 mov         ecx,dword ptr [rsp+4]
 mov         dword ptr [rsp],ecx
 mov         dword ptr [rsp+4],eax
 mov         eax,dword ptr [rsp+4]
 add         eax,dword ptr [rsp]
 add         rsp,38h
 ret

Into:

 sub         rsp,20h
 mov         eax,2
 add         eax,1
 add         rsp,20h
 ret

Which is identical to the CArray test's generated code.

Bug: b/179279298
Change-Id: Ie45261f2ac783bdc22fee06adf03ebd588f3ebc3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52428
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>