[X86] When lowering uniform shifts, use X86ISD::VZEXT instead of using a...

[X86] When lowering uniform shifts, use X86ISD::VZEXT instead of using a ZERO_EXTEND_VECTOR_INREG. If we emit the ZERO_EXTEND_VECTOR_INREG too late it doesn't get lowered properly and makes it through to isel and fails.

Fixes PR31593.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291535 91177308-0d34-0410-b5e6-96231b3b80d8