[AArch64][GlobalISel] Constrain G_CONSTANT_FOLD_BARRIER operand register classes (#177997)

[AArch64][GlobalISel] Constrain G_CONSTANT_FOLD_BARRIER operand

Instruction selection is lowering:

  bb.1:
    %6:gpr(s64) = G_CONSTANT i64 457873110
    ...
  bb.2:
    %12:gpr(s64) = G_CONSTANT_FOLD_BARRIER %6
    %24:gpr(s64) = G_CONSTANT i64 0
    %13:gpr(s64) = G_AND %24, %12
    ...

to:

  %13:gpr64 = ANDXrr %24:gpr64, %6:gpr64sp'

which is causing the verifier to fail with:

  Expected a GPR64 register, but got a GPR64sp register

the reason for this is there are multiple G_CONSTANT_FOLD_BARRIER ops
with the same input register:

  Select:  %14:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr
  Erasing:   %14:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64sp
  --
  Select:  %12:gpr64(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64sp
  Erasing:   %12:gpr64(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64
  --
  Select:  %7:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64
  Erasing:   %7:gpr64sp(s64) = G

and the middle one used by the AND gets overridden by last and becomes
invalid.

this patch fixes this by constraining the register class such that the
middle one gets constrained to the subset of gpr64sp and gpr64, i.e.
gpr64common:

  Select:  %14:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr
  Erasing:   %14:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64sp
  --
  Select:  %12:gpr64(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64sp
  Erasing:   %12:gpr64(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64common
  --
  Select:  %7:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64common
  Erasing:   %7:gpr64sp(s64) = G_CONSTANT_FOLD_BARRIER %6:gpr64common

canReplaceReg doesnt handle when both regs already have classes so the
assert is removed.

Fixes #166563.