[CodeGen] Enhance machine PHIs optimization

Summary:
Make machine PHIs optimization to work for single value register taken from
several different copies. This is the first step to fix PR38917. This change
allows to get rid of redundant PHIs (see opt_phis2.mir test) to make
the subsequent optimizations (like CSE) possible and simpler.

For instance, before this patch the code like this:

%b = COPY %z
...
%a = PHI %bb1, %a; %bb2, %b
could be optimized to:

%a = %b
but the code like this:

%c = COPY %z
...
%b = COPY %z
...
%a = PHI %bb1, %a; %bb2, %b; %bb3, %c
would remain unchanged.
With this patch the latter case will be optimized:

%a = %z```.

Committed on behalf of: Anton Afanasyev anton.a.afanasyev@gmail.com

Reviewers: RKSimon, MatzeB

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54839

llvm-svn: 349271