[GlobalISel][RISCV][TableGen] Teach GlobalISelEmitter about HwMode.

Similar to SelectionDAG, this patch treats HwMode as an additional
predicate that needs to be satisfied for GIM_CheckFeatures.

The existing predicate passes around Record * that point to predicate
records. While HwMode expansion creates a string that needs to be
checked.

Each HwMode predicate string is uniqued by a new map that assigns
it an index. Each Rule stores the index, or -1 if HwMode doesn't
apply.

The HwMode indices each create a new Predicate feature bit and the
check string from the HwMode is used to set the feature bit.
GIM_CheckFeatures is emitted when the rule has Predicates or the HwModeIdx
is not -1.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D158660