Remove check for single use in ShrinkDemandedConstant

This removes check for single use from general ShrinkDemandedConstant
to the BE because of the AArch64 regression after D56289/rL350475.

After several hours of experiments I did not come up with a testcase
failing on any other targets if check is not performed.

Moreover, direct call to ShrinkDemandedConstant is not really needed
and superceed by SimplifyDemandedBits.

Differential Revision:

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed