x86-64: Code size improvement for MethodLoadKind::kDirectAddress.

Instead of movq(), use Load64BitValue() which uses MOVL
for 32-bit unsigned values. For pointers to low 2GiB,
this saves 2 bytes, for pointers to high 2GiB (of 32-bit
range) it saves 5 bytes.

Test: testrunner.py --host
Change-Id: I437be6e7486329da53674b051f48a0afc4b3a5a5