Loosen StoreIndexedInstr::MakeLocationSummary RAX requirement

sdk/runtime/vm/compiler/backend/il_x64.cc

Lines 1920 to 1922 in a4929ae

	// TODO(fschneider): Add location constraint for byte registers (RAX,
	// RBX, RCX, RDX) instead of using a fixed register.
	locs->set_in(2, LocationFixedRegisterOrSmiConstant(value(), RAX));

The above RAX restriction is potentially no longer needed as of https://dart-review.googlesource.com/c/sdk/+/237690.
Dart's x64 assembly used not be able to express movs to byte registers above the first four, but after the above fix that is no longer the case.

We should check whether there are any other blockers and otherwise remove this TODO.