[PowerPC][GIsel] Materialize i64 constants.

Adds support for i64 constant. It uses the same pattern-based
approach as in SDAG (see PPCISelDAGToDAG::selectI64ImmDirect(),
PPCISelDAGToDAG::selectI64Imm()). It does not support the
prefixed instructions.

Reviewed By: arsenm, tschuett

Differential Revision: https://reviews.llvm.org/D140119