[X86] X86InstrFragments.td - replace def32 with something that will fail-safe

[RKSimon]

This came up recently in #114360 - the def32 pattern to detect implicit i32->i64 zero extension can get things wrong under certain circumstances depending on ordering in DAGISel and assume the upper 32-bit are already zero when they aren't.

We might be better off just having a pass that removes unnecessary zext nodes later on.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

This came up recently in #114360 - the def32 pattern to detect implicit i32->i64 zero extension can get things wrong under certain circumstances depending on ordering in DAGISel and assume the upper 32-bit are already zero when they aren't.

We might be better off just having a pass that removes unnecessary zext nodes later on.