Add an explicit bool operator to SDValue to make it easier to test for
a non-null node. In particular, this makes it easier to use condition
variables with SDValues, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212323 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Generalize BuildVectorSDNode::getConstantSplatValue to work for
any constant, constant FP, or undef splat and to tolerate any undef
lanes in a splat, then replace all uses of isSplatVector in X86's
lowering with it.

This fixes issues where undef lanes in an otherwise splat vector would
prevent the splat logic from firing. It is a touch more awkward to use
this interface, but it is much more accurate. Suggestions for better
interface structuring welcome.

With this fix, the code generated with the widening legalization
strategy for widen_cast-4.ll is *dramatically* improved as the special
lowering strategies for a v16i8 SRA kick in even though the high lanes
are undef.

We also get a slightly different choice for broadcasting an aligned
memory location, and use vpshufd instead of vbroadcastss. This looks
like a minor win for pipelining and domain crossing, but a minor loss
for the number of micro-ops. I suspect its a wash, but folks can easily
tweak the lowering if they want.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212324 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Move some check lines to be slightly easier for me to find.

(meant to put this cleanup in the previous patch, sorry)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212325 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Relax the line in this check to pacify build bots.

I still don't love testing the comments, but its the only sane way to
check shuffle instructions...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212326 91177308-0d34-0410-b5e6-96231b3b80d8