DAG: avoid duplicated truncating for sign extended operand

Summary:
When performing cmp for EQ/NE and the operand is sign extended, we can
avoid the truncaton if the bits to be tested are no less than origianl
bits.

Reviewers: eli.friedman

Subscribers: eli.friedman, aemerson, nemanjai, t.p.northover, llvm-commits

Differential Revision: https://reviews.llvm.org/D22933

llvm-svn: 277252