[BitcodeReader] Fix DelayedShuffle handling for ConstantExpr shuffles.

The indexing was messed up, so the result was completely broken.

Shuffle constant exprs are rare in practice; without vscale types,
constant folding generally elminates them. So sort of hard to trip over.

Fixes regression from D72467.

(Recommitting after fix for memory leak.)

Differential Revision: https://reviews.llvm.org/D80330