X86: Fix X86CallFrameOptimization to search for the COPY StackPointer

SelectionDAG inserts a copy of ESP into a virtual register.
X86CallFrameOptimization assumed that the COPY, if present, is always
right after the call-frame setup instruction (ADJCALLSTACKDOWN). This was a
wrong assumption as the COPY can be located anywhere between the call-frame setup
instruction and its first use. If the COPY happened to be located in a different
location than what X86CallFrameOptimization assumed, visiting it while
processing the call chain would lead to a conservative bail-out.

The fix is quite straightfoward, scan ahead for the stack-pointer copy and make note
of it so it can be ignored while processing the call chain.

Fixes pr34903

Differential Revision: https://reviews.llvm.org/D38730

llvm-svn: 316416