[AVX512] Fix i256mem->f256mem typo in VINSERTF64x4rm

Just like in the case of extracts, the refactoring is uncovering some typos in
the code.
[AVX512] Refactor subvector inserts

No functional change.

Very similar to the extract refactoring I did in r218478.

Compared X86.td.expanded before and after.
[AVX512] Pull pattern for subvector insert into the instruction definition

No functional change intended.

Very similar to the change I made for subvector extract in r218480.

test/CodeGen/X86/avx512-insert-extract.ll covers this.