[llvm-mca] Add the ability to mark register reads/writes associated with...

[llvm-mca] Add the ability to mark register reads/writes associated with dep-breaking instructions. NFCI

This patch adds two new boolean fields:
  - Field `ReadState::IndependentFromDef`.
  - Field `WriteState::WritesZero`.

Field `IndependentFromDef` is set for ReadState objects associated with
dependency-breaking instructions. It is used by the simulator when updating data
dependencies between registers.

Field `WritesZero` is set by WriteState objects associated with dependency
breaking zero-idiom instructions. It helps the PRF identify which writes don't
consume any physical registers.

llvm-svn: 342483