[X86] Remove dead declaration convertArgMovsToPushes, NFC

This was dead when it landed in r252578. We have this functionality, if
not for stack probe calls, but for regular calls in
X86CallFrameOptimization.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314845 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed