[InstCombine][SSE] Demanded vector elements for scalar intrinsics (Part 1 of 2)

This patch improves support for determining the demanded vector elements through SSE scalar intrinsics:

1 - recognise that we only need the lowest element of the second input for binary scalar operations (and all the elements of the first input)

2 - recognise that the roundss/roundsd intrinsics use the lowest element of the second input and the remaining elements from the first input

Differential Revision: http://reviews.llvm.org/D17490

llvm-svn: 267356