[X86] Prevent folding stack reloads into instructions in hasUndefRegUpdate.

[X86] Prevent folding stack reloads into instructions in hasUndefRegUpdate.

An earlier commit prevented folds from the peephole pass by checking for IMPLICIT_DEF. But later in the pipeline IMPLICIT_DEF just becomes and Undef flag on the input register so we need to check for that case too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334848 91177308-0d34-0410-b5e6-96231b3b80d8