[X86] Correct v4f32->v2i64 cvt(t)ps2(u)qq memory isel patterns

These instructions only read 64-bits of memory so we shouldn't
allow a full vector width load to be pattern matched in case it
is marked volatile.

Instead allow vzload or scalar_to_vector+load.

Also add a DAG combine to turn full vector loads into vzload when
used by one of these instructions if the load isn't volatile.

This fixes another case for PR42079

llvm-svn: 364838