[X86] Fix latent bug in sibcall eligibility logic

The X86 tail call eligibility logic was correct when it was written, but
the addition of inalloca and argument copy elision broke its
assumptions. It was assuming that fixed stack objects were immutable.

Currently, we aim to emit a tail call if no arguments have to be
re-arranged in memory. This code would trace the outgoing argument
values back to check if they are loads from an incoming stack object.
If the stack argument is immutable, then we won't need to store it back
to the stack when we tail call.

Fortunately, stack objects track their mutability, so we can just make
the obvious check to fix the bug.

This was http://crbug.com/749826

llvm-svn: 309343