[GlobalISel] Change ConstantFoldVectorBinop to return vector of APInt

Previously it built MIR for the results and returned a Register.

This avoids building constants for earlier elements of the vector if
later elements will fail to fold, and allows CSEMIRBuilder::buildInstr
to avoid unconditionally building a copy from the result.

Use a new helper function MachineIRBuilder::buildBuildVectorConstant
to build a G_BUILD_VECTOR of G_CONSTANTs.

Differential Revision: https://reviews.llvm.org/D117758