[RegAllocFast] properly handle STATEPOINT instruction.

STATEPOINT is a fancy and complex pseudo instruction which
has both tied defs and regmask operand.

Basic FastRA algorithm is as follows:

1. Mark registers used by defs as free
2. If instruction has regmask operand displace clobbered registers
   according to regmask.
3. Assign registers for use operands.

In case of tied defs step 1 is replaced with allocation of registers
for them. But regmask is still processed, which may displace already
allocated registers. As a result, tied use and def will get assigned
to different registers.

This patch makes FastRA to process instruction's RegMask (if any) when
checking for physical registers interference.
That way tied operands won't get registers clobbered by regmask.

Reviewed By: arsenm, skatkov
Differential Revision: https://reviews.llvm.org/D99284