[RegisterCoalescer] Add new subtarget hook allowing targets to opt-out of coalescing.

The coalescer is very aggressive at propagating constraints on the register classes, and the register allocator doesn’t know how to split sub-registers later to recover. This patch provides an escape valve for targets that encounter this problem to limit coalescing.

This patch also implements such for ARM to lower register pressure when using lots of large register classes. This works around PR18825.