[InstCombine] Fix context for multi-use demanded bits simplification

When simplifying a multi-use root value, the demanded bits were
reset to full, but we also need to reset the context extract. To
make this convenient (without requiring by-value passing of
SimplifyQuery), move the logic that that handles constants and
dispatches to SimplifyDemandedUseBits/SimplifyMultipleUseDemandedBits
into SimplifyDemandedBits. The SimplifyDemandedInstructionBits
caller starts with full demanded bits and an appropriate context
anyway.

The different context instruction does mean that the ephemeral
value protection no longer triggers in some cases, as the changes
to assume tests show.

An alternative, which I will explore in a followup, is to always
use SimplifyMultipleUseDemandedBits() -- the previous root special
case is only really intended for SimplifyDemandedInstructionBits(),
which now no longer shares this code path.

Fixes https://github.com/llvm/llvm-project/issues/97330.

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