[Pipeliner] copyToPhi DAG Mutation to improve scheduling.

In a loop, create artificial dependences between the source of a
COPY/REG_SEQUENCE to the use in next iteration.

Eg:
SRC ----Data Dep--> COPY
COPY ---Anti Dep--> PHI (implies, to be used in next iteration)
PHI ----Data Dep--> USE

This patches creates
USE ----Artificial Dep---> SRC

This will effectively schedule the COPY late to eliminate additional copies.
Before this patch, the schedule can be
SRC, COPY, USE : The COPY is used in next iteration and it needs to be
preserved.

After this patch, the schedule can be
USE, SRC, COPY : The COPY is used in next iteration and the live interval is
reduced.

Differential Revision: https://reviews.llvm.org/D53303

llvm-svn: 344748