[Powerpc] set instruction count as lsr first priority of lsr.

[Powerpc] set instruction count as lsr first priority of lsr.

On Powerpc, set instruction count as lsr first priority of lsr by default.
Add an option ppc-lsr-no-insns-cost to return back to default lsr cost model.

Reviewed By: steven.zhang, jsji

Differential Revision: https://reviews.llvm.org/D72683