[GlobalISel] Add matchers for constant splat.

This change exposes isBuildVectorConstantSplat() to the llvm namespace
and uses it to implement the constant splat versions of
m_SpecificICst().

CombinerHelper::matchOrShiftToFunnelShift() can now work with vector
types and CombinerHelper::matchMulOBy2()'s match for a constant splat is
simplified.

Differential Revision: https://reviews.llvm.org/D114625