[PowerPC] Add an ISEL pattern for Mul with Imm.

Summary: This patch try to do the following transformation if the multiplier doen't fit int16:
			(mul X, c1 << c2) -> (rldicr (mulli X, c1) c2)

Reviewed By: jsji, steven.zhang

Differential Revision: https://reviews.llvm.org/D87384