[AArch64] Fix legalization of i128 ctpop without neon

If neon is disabled, LowerCTPOP will return SDValue() to indicate
that normal legalization should be used. However, ReplaceNodeResults
does not check for this and pushes the empty SDValue() onto the
result vector, which will subsequently result in a crash.

Differential Revision: https://reviews.llvm.org/D93825