[InstCombine] Merge together the SimplifyDemandedUseBits implementations for ZExt and Trunc. NFC

While there avoid resizing the DemandedMask twice. Make a copy into a separate variable instead. This potentially removes an allocation on large bit widths.

With the use of the zextOrTrunc methods on APInt and KnownBits these can be made almost source identical. The only difference is the zero of the upper bits for ZExt. This is similar to how its done in computeKnownBits in ValueTracking.

llvm-svn: 303791