[X86][Codegen] PR51615: don't replace wide volatile load with narrow broadcast-from-memory

Even though https://bugs.llvm.org/show_bug.cgi?id=51615
appears to be introduced by D105390, the fix lies here.

We can not replace a wide volatile load with a broadcast-from-memory,
because that would narrow the load, which isn't legal for volatiles.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D108757