[X86] Split 512-bit vector extends from types other than vXi1 out of...

[X86] Split 512-bit vector extends from types other than vXi1 out of LowerZERO_EXTEND_AVX512/LowerSIGN_EXTEND_AVX512. NFCI

Most of the code in these routines is for handling extends from vXi1 types. The 512-bit handling for other extends is very much like the AVX2 code. So make the special routines just do vXi1 types and move the other 512-bit handling to the place that handles AVX2.

llvm-svn: 319878