[MCA] Add the ability to compute critical register dependency of an instruction.

[MCA] Add the ability to compute critical register dependency of an instruction.

This patch adds the methods `getCriticalRegDep()` and `computeCriticalRegDep()` to
class InstructionBase.
The goal is to allow users to obtain information about the critical register
dependency that most affects the latency of an instruction.

These methods are currently unused. However, the long term plan is to use them
in order to allow the computation of a critical-path as part of the bottleneck
analysis. So, this is yet another step towards fixing PR37494.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361509 91177308-0d34-0410-b5e6-96231b3b80d8