[LAA] NFC: Rename [get]MaxSafeRegisterWidth -> [get]MaxSafeVectorWidthInBits

MaxSafeRegisterWidth is a misnomer since it actually returns the maximum
safe vector width. Register suggests it relates directly to a physical
register where it could be a vector spanning one or more physical
registers.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D91727