[DAG] SimplifyDemandedBits - don't early-out for multiple use values

SimplifyDemandedBits currently early-outs for multi-use values beyond the root node (just returning the knownbits), which is missing a number of optimizations as there are plenty of cases where we can still simplify when initially demanding all elements/bits.

@lenary has confirmed that the test cases in aea-erratum-fix.ll need refactoring and the current increase codegen is not a major concern.

Differential Revision: https://reviews.llvm.org/D129765