[AVX512] Correct isExtractSubvectorCheap so that it will return the correct...

[AVX512] Correct isExtractSubvectorCheap so that it will return the correct answers for extracting 128-bits from a 512-bit vector and for mask registers.

Previously it would not return true for extracting either of the upper quarters of a 512-bit registers.

For mask registers we support extracting anything from index 0. And otherwise we only support extracting the upper half of a register.

Differential Revision: https://reviews.llvm.org/D36638

llvm-svn: 310794