[InstCombine][SSE] Demanded vector elements for scalar intrinsics (Part 2 of 2)

Split from D17490. This patch improves support for determining the demanded vector elements through SSE scalar intrinsics:

1 - demanded vector element support for unary and some extra binary scalar intrinsics (RCP/RSQRT/SQRT/FRCZ and ADD/CMP/DIV/ROUND).

2 - addss/addsd get simplified to a fadd call if we aren't interested in the pass through elements

3 - if we don't need the lowest element of a scalar operation then just use the first argument (the pass through elements) directly

We can add support for propagating demanded elements through any equivalent packed SSE intrinsics in a future patch (these wouldn't use the pass through patterns).

Differential Revision: http://reviews.llvm.org/D19318

llvm-svn: 267357