[BDCE] SExt -> ZExt when no sign bits is used and instruction has multiple uses

Summary: This allows to convert any SExt to a ZExt when we know none of the extended bits are used, specially in cases where there are multiple uses of the value.

Reviewers: dmgreen, eli.friedman, spatel, lebedev.ri, nikic

Reviewed By: lebedev.ri, nikic

Subscribers: hiraditya, dmgreen, craig.topper, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60413