[AArch64][GlobalISel] Constrain reg operands in selectBrJT

This was causing a machine verifier failure on the test suite.

Make sure that we don't end up with a weird register class here.

Failure for reference:

*** Bad machine code: Illegal virtual register for instruction ***
- function:    check_constrain
- basic block: %bb.1  (0x7f8b70839f80)
- instruction: early-clobber %6:gpr64, early-clobber %7:gpr64sp =
  JumpTableDest32 %5:gpr64, %1:gpr64sp, %jump-table.0
- operand 3:   %1:gpr64sp
Expected a GPR64 register, but got a GPR64sp register

Differential Revision: https://reviews.llvm.org/D77349