[X86] Complete preservation of !pcsections in X86ISelLowering

https://reviews.llvm.org/D130883 introduced MIMetadata to simplify
metadata propagation (DebugLoc and PCSections).

However, we're currently still permitting implicit conversion of
DebugLoc to MIMetadata, to allow for a gradual transition and let the
old code work as-is.

This manifests in lost !pcsections metadata for X86-specific lowerings.
For example, 128-bit atomics.

Fix the situation for X86ISelLowering by converting all BuildMI() calls
to use an explicitly constructed MIMetadata.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D154986