[RegisterCoalescer] fix potential use of undef value. NFC

Summary:
Fixes a warning produced from scan-build (llvm.org/reports/scan-build/),
further warnings found by annotation isMoveInstr [[nodiscard]].

isMoveInstr potentially does not assign to its parameters, so if they
were uninitialized, they will potentially stay uninitialized.  It seems
most call sites pass references to uninitialized values, then use them
without checking the return value.

Reviewers: wmi

Reviewed By: wmi

Subscribers: MatzeB, qcolombet, hiraditya, tpr, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62109

llvm-svn: 362265