[AArch64][GlobalISel] Tail call memory intrinsics

Because memory intrinsics are handled differently than other calls, we need to
check them for tail call eligiblity in the legalizer. This allows us to still
inline them when it's beneficial to do so, but also tail call when possible.

This adds simple tail calling support for when the intrinsic is followed by a
return.

It ports the attribute checks from `TargetLowering::isInTailCallPosition` into
a similarly-named function in LegalizerHelper.cpp. The target-specific
`isUsedByReturnOnly` hook is not ported here.

Update tailcall-mem-intrinsics.ll to show that GlobalISel can now tail call
memory intrinsics.

Update legalize-memcpy-et-al.mir to have a case where we don't tail call.

Differential Revision: https://reviews.llvm.org/D67566

llvm-svn: 371893