Substantially improve code generation for address exposed locals (aka fixed

Substantially improve code generation for address exposed locals (aka fixed
sized allocas in the entry block).  Instead of generating code like this:

entry:
  reg1024 = ESP+1234
... (much later)
  *reg1024 = 17

Generate code that looks like this:
entry:
  (no code generated)
... (much later)
  t = ESP+1234
  *t = 17

The advantage being that we DRAMATICALLY reduce the register pressure for these
silly temporaries (they were all being spilled to the stack, resulting in very
silly code).  This is actually a manual implementation of rematerialization :)

I have a patch to fold the alloca address computation into loads & stores, which
will make this much better still, but just getting this right took way too much time
and I'm sleepy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13554 91177308-0d34-0410-b5e6-96231b3b80d8