FastISel: For calls, prefer using the callee's address as a constant

over having it in a register. And wait until after checking type
legality before requesting that the callee address be placed in a
register. Also, fix support for calls with void return type.

This speeds up fast-isel isel time by about 15% and reduces
instruction counts by about 3% overall on certain testcases. It also
changes many indirect calls to direct calls.

llvm-svn: 56292