[AArch64][MachineCombiner] Use MIMetadata to copy pcsections metadata to reassociated instructions.

D134260/D138107 exposed that the MachineCombiner was not copying
pcsections metadata where it should. This patch switches the MIBuild
methods to use MIMetadata that can copy the debug loc and pcsections at
the same time.

Differential Revision: https://reviews.llvm.org/D138112