[X86] Auto-upgrade AVX1 vbroadcast intrinsics

They are replaced with the same IR that is generated for the
vector-initializers in avxintrin.h.

The test verifies that we get back the original instruction.  I haven't seen
this approach to be used in other auto-upgrade tests (i.e. llc + FileCheck)
but I think it's the most direct way to test this case.  I believe this should
work because llc upgrades calls during parsing.  (Other tests mostly check
that assembling and disassembling yields the upgraded IR.)
[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.