[InstCombine] remove flop from lerp patterns

(Y * (1.0 - Z)) + (X * Z) -->
Y - (Y * Z) + (X * Z) -->
Y + Z * (X - Y)

This is part of solving:
https://bugs.llvm.org/show_bug.cgi?id=42716

Factoring eliminates an instruction, so that should be a good canonicalization.
The potential conversion to FMA would be handled by the backend based on target
capabilities.

Differential Revision: https://reviews.llvm.org/D65305

llvm-svn: 367101