[x86] try harder to use broadcast to load a scalar into vector reg

This is a preliminary step for a preliminary step for D50992. 
I noticed that x86 often misses chances to load a scalar directly 
into a vector register.

So this patch is just allowing more of those cases to match a 
broadcast op in lowerBuildVectorAsBroadcast(). The old code comment 
said it doesn't make sense to use a broadcast when we're loading a 
single element and everything else is undef, but I think that's the 
best case in the improved tests in insert-loaded-scalar.ll. We avoid 
scalar-to-vector-register move and/or less efficient shuffling.

Note that there are some existing types that were already producing 
a broadcast, but that happens semi-accidentally. Ie, it's not 
happening as part of lowerBuildVectorAsBroadcast(). The build vector 
gets expanded into load + shuffle, and then shuffle lowering produces 
the broadcast.

Description of the other test diffs:
1. avx-basic.ll - replacing load+shufle is a win.
2. sse3-avx-addsub-2.ll - vmovddup vs. vbroadcastss is neutral
3. sse41.ll - don't care - we convert that intrinsic to generic IR now, so this test is deprecated
4. vector-shuffle-128-v8.ll / vector-shuffle-256-v16.ll - pshufb alternatives with an extra instruction are not obviously bad

Differential Revision: https://reviews.llvm.org/D51125

llvm-svn: 340685