[RegAlloc] Constrain rematted regclass to use (llvm#164386)

When rematting we create a new virtual register with the original def's
register class. However the use may have a different register class if
the interval is split, which means we end up with an invalid register
class.

This fixes llvm#164181 by constraining the newly created register to the
use's register class.

The test case is reduced as far as it goes. Because this test requires
us to reach a certain amount of register pressure in certain conditions
I'm not sure if there's an easy way to handwrite this scenario.

Author: lukel97

llvm/lib/CodeGen/InlineSpiller.cpp

@@ -721,6 +721,9 @@ bool InlineSpiller::reMaterializeFor(LiveInterval &VirtReg, MachineInstr &MI) {
   // Allocate a new register for the remat.
   Register NewVReg = Edit->createFrom(Original);
 
+  // Constrain it to the register class of MI.
+  MRI.constrainRegClass(NewVReg, MRI.getRegClass(VirtReg.reg()));
+
   // Finally we can rematerialize OrigMI before MI.
   SlotIndex DefIdx =
       Edit->rematerializeAt(*MI.getParent(), MI, NewVReg, RM, TRI);