[X86] Remove AVX1 vbroadcast intrinsics

[X86] Remove AVX1 vbroadcast intrinsics

The corresponding CFE patch replaces these intrinsics with vector initializers
in avxintrin.h.  This patch removes the LLVM intrinsics from the backend.

We now stop lowering at X86ISD::VBROADCAST custom node rather than lowering
that further to the intrinsics.

The patch only changes VBROADCASTS* and leaves VBROADCAST[FI]128 to continue
to use intrinsics.  As explained in the CFE patch, the reason is that we
currently don't generate as good code for them without the intrinsics.

CodeGen/X86/avx-vbroadcast.ll already provides coverage for this change.  It
checks that for a series of insertelements we generate the appropriate
vbroadcast instruction.

Also verified that there was no assembly change in the test-suite before and
after this patch.

llvm-svn: 209864