[TableGen] Support encoding per-HwMode

Much like ValueTypeByHwMode/RegInfoByHwMode, this patch allows targets
to modify an instruction's encoding based on HwMode. When the
EncodingInfos field is non-empty the Inst and Size fields of the Instruction
are ignored and taken from EncodingInfos instead.

As part of this promote getHwMode() from TargetSubtargetInfo to MCSubtargetInfo.

This is NFC for all existing targets - new code is generated only if targets
use EncodingByHwMode.

llvm-svn: 372320