[CodeGen] Don't assume that fixed stack objects are aligned in a stack-realigned...

[CodeGen] Don't assume that fixed stack objects are aligned in a stack-realigned function.

Summary:
After we make the adjustment, we can assume that for local allocas, but
not for stack parameters, the return address, or any other fixed stack
object (which has a negative offset and therefore lies prior to the
adjusted SP).

Fixes PR26662.

Reviewers: hfinkel, qcolombet, rnk

Subscribers: rnk, llvm-commits

Differential Revision: http://reviews.llvm.org/D18471

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265886 91177308-0d34-0410-b5e6-96231b3b80d8