[InstCombine] Fold for masked gather when loading the same value each time.

This patch checks in the masked gather when the first operand value is a
splat and the mask is all one, because the masked gather is reloading the
same value each time. This patch replaces this pattern of masked gather by
a scalar load of the value and splats it in a vector.

Differential Revision: https://reviews.llvm.org/D115726