AMDGPU: Make SIInsertWaits about a factor of 4 faster

This was the slowest target custom pass and was spending 80%
of the time in getMinimalPhysRegClass which was called
for every register operand.

Try to use the statically known register class when possible from
the instruction's MCOperandInfo. There are a few pseudo instructions
which are not well behaved with unknown register classes which still
require the expensive physical register class search.

There are a few other possibilities for making this even faster,
such as not inspecting implicit operands. For now those are checked
because it is technically possible to have a scalar load into
exec or vcc which can be implicitly used.

llvm-svn: 249079