[X86] Teach combineToExtendBoolVectorInReg to create opportunities for using broadcast load instructions.
If we're inserting a scalar that is smaller than the element
size of the final VT, the value of the extra bits doesn't matter.
Previously we any_extended in the scalar domain before inserting.
This patch changes this to use a broadcast of the original
scalar type and then a bitcast to the final type. This might
enable the use of a broadcast load.
Differential Revision: https://reviews.llvm.org/D75835