FeaturePMU was created in AArch64 to accommodate one missing system
register, PMMIR_EL1, in commit ffcd7698aea7bcbb2b4edffc484793e1ff47b85d.
However, the Performance Monitors extension already had a target
feature, which is called FeaturePerfMon. Therefore, FeaturePMU is
redundant.
This patch removes FeaturePMU and merges its contents into
FeaturePerfMon.
Reviewed By: dnsampaio
Differential Revision: https://reviews.llvm.org/D109246
79f9c79aaf