ARM: Clean up fast-isel machine verifier errors.

Lots of machine verifier errors result from using a plain GPR regclass
for incoming argument copies. A more restrictive rGPR class is more
appropriate since it more accurately represents what's happening, plus
it lines up better with isel later on so the verifier is happier.
Reduces the number of ARM fast-isel tests not running with the verifier
enabled by over half.

rdar://12594152
ARM: Fix more fast-isel verifier failures.

Teach the generic instruction selection helper functions to constrain
the register classes of their input operands. For non-physical register
references, the generic code needs to be careful not to mess that up
when replacing references to result registers. As the comment indicates
for MachineRegisterInfo::replaceRegWith(), it's important to call
constrainRegClass() first.

rdar://12594152
ARM: Fast-isel register class constrain for extends.

Properly constrain the operand register class for instructions used
in [sz]ext expansion. Update more tests to use the verifier now that
we're getting the register classes correct.

rdar://12594152
ARM: Properly constrain comparison fastisel register classes.

Ongoing 'make the verifier happy' improvements to ARM fast-isel.

rdar://12594152