[MCA] Correctly update register definitions in the PRF after move elimination.

This patch fixes a bug where register writes performed by optimizable register
moves were sometimes wrongly treated like partial register updates.  Before this
patch, llvm-mca wrongly predicted a 1.50 IPC for test reg-move-elimination-6.s
(added by this patch).  With this patch, llvm-mca correctly updates the register
defintions in the PRF, and the IPC for that test is now correctly reported as 2.

llvm-svn: 354271