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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319878 91177308-0d34-0410-b5e6-96231b3b80d8