[AArch64][GISel] constrain regclass for 128->64 copy

When selecting G_EXTRACT to COPY for extracting a 64-bit GPR from
a 128-bit register pair (XSeqPair) we know enough to constrain the
destination register class to gpr64. Without this it may have only
a register bank and some copy elimination code would assert while
assuming that a register class existed.

The register class has to be set explicitly because we might hit the
COPY -> COPY case where register class can't be inferred.

This would cause the following to crash in selection, where the store
is commented (otherwise the store constrains the register class):

  define dso_local i128 @load_atomic_i128_unordered(i128* %p) {
    %pair = cmpxchg i128* %p, i128 0, i128 0 acquire acquire
    %val = extractvalue { i128, i1 } %pair, 0
    ; store i128 %val, i128* %p
    ret i128 %val
  }

Differential Revision: https://reviews.llvm.org/D132665