Don't nil check non-nil class receiver of AArch64 stret calls.

I randomly came across this difference between AArch64 and other targets:
on the latter, we don't emit nil checks for known non-nil class method
calls thanks to r247350, but we still do for AArch64 stret calls.

They use different code paths, because those are special, as they go
through the regular msgSend, not the msgSend*_stret variants.

llvm-svn: 249205