[InstCombine] Simplify binops that are only used by a select and are fed by a...

[InstCombine] Simplify binops that are only used by a select and are fed by a select with the same condition.

Summary:
This patch optimizes a binop sandwiched between 2 selects with the same condition. Since we know its only used by the select we can propagate the appropriate input value from the earlier select.

As I'm writing this I realize I may need to avoid doing this for division in case the select was protecting a divide by zero?

Reviewers: spatel, majnemer

Reviewed By: majnemer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39999

llvm-svn: 318267