[X86] Disable a DAG combine to allow packed AVX512DQ instructions to be...

[X86] Disable a DAG combine to allow packed AVX512DQ instructions to be consistently used for i64->float/double conversions.

Summary: We already get this right if the i64 didn't come from a load.

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47439

llvm-svn: 333393