[X86] Fix a shrink-wrapping miscompile around __chkstk

__chkstk clobbers EAX. If EAX is live across the prologue, then we have
to take extra steps to save it. We already had code to do this if EAX
was a register parameter. This change adapts it to work when shrink
wrapping is used.

llvm-svn: 261039