[NFC][Regalloc] Pass VirtRegMap by reference.

[NFC][Regalloc] Pass VirtRegMap by reference.

It's never null - the reason it's modeled as a pointer is because the
pass can't init it in its ctor. Passing by ref simplifies the code, too,
as the null checks were unnecessary complexity.

Differential Revision: https://reviews.llvm.org/D89171