[GlobalISel] Copy the implementation of SubtargetFeature and use it for PredicateBitset.

PredicateBitset currently uses std::bitset, but std::bitset doesn't
have a constexpr constructor or any constexpr methods until C++23.
Each target that supports GlobalIsel has as an array of PredicateBitset
objects that currently use a global constructor.

SubtargetFeature used by the MC layer for feature bits, has its own
implementation of std::bitset that has constexpr constructor and methods
that provides all the capabilities that PredicateBitset needs.

This patch copies the implementation from SubtargetFeature, makes
it a template class, and puts it in ADT. I'll migrate SubtargetFeature
in a separate patch. Adapting all existing users to it being a template
was distracting from the goal of this patch.

This reduces the binary size of llc built with gcc 8.5.0 on my local
build by ~15k.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D158576

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