GlobalISel: Add default implementation of assignValueToReg

Refactor insertion of the asserting ops. This enables using them for
AMDGPU.

This code should essentially be the same for every target. Mips, X86
and ARM all have different code there now, but this seems to be an
accident. The assignment functions are called with different types
than they would be in the DAG, so this is all likely an assortment of
hacks to get around that.