GlobalISel: Split ValueHandler into assignment and emission classes

Currently the ValueHandler handles both selecting the type and
location for arguments, as well as inserting instructions needed to
handle them. Split this so that the determination of the argument
handling is independent of the function state. Currently the checks
for tail call compatibility do not follow the full assignment logic,
so it misses cases where arguments require nontrivial legalization.

This should help avoid targets ending up in a buggy state where the
argument evaluation may change in different contexts.