[PreISelIntrinsicLowering] Use TLI for correct function

We should query the subtarget of the calling function, not of the
intrinsic.

This probably makes no functional difference (as libcalls are
unlikely to vary across subtargets), but fixes minor compile-time
regressions from unnecessary subtarget instantiations.

Followup to D157567.

Differential Revision: https://reviews.llvm.org/D157848