Revert "[X86][AVX] Add getBROADCAST_LOAD helper function. NFCI."

This reverts commit 1cfecf4f.

This commit broke LLVM code generated through XLA by removing a
conditional on Ld->getExtensionType() == ISD::NON_EXTLOAD

This is not a perfect revert. The new function is left as other uses of
it exist now.