[GlobalISel] Add buildMerge with SrcOp initializer list

[GlobalISel] Add buildMerge with SrcOp initializer list

Allows more flexible use of buildMerge in places where
use operands are available as SrcOp since it does not
require explicit conversion to Register.
Simplify code with new buildMerge.

Differential Revision: https://reviews.llvm.org/D74223