Revert "[SLP]Improve tryToGatherExtractElements by using per-register analysis."

This reverts commit 3e6d7c6d.

That commit caused miscompilation of ffmpeg's libavcodec/vp9dsp_8bpp.o
on aarch64; the file still compiles correctly, but no longer produces
the right result - see https://reviews.llvm.org/D148855#4655968
for details.