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.

llvm-svn: 13554