[GTO] Update struct field indices after localizing operands

[tlively]

PR #9043 ("Avoid newly exposing prototypes in GTO") introduced prepending
an i8 placeholder field at index 0 of exposed no-proto descriptors when
their first field becomes prototype-exposing. When all fields of such a
descriptor are kept, the new index of the last field equals the old
struct's fields.size().

Because updateInstructions runs before updateTypes (so expression
heap types still refer to the pre-optimization Struct definitions), and
FieldRemover is a PostWalker, child struct.get, struct.rmw, and
struct.cmpxchg instructions had their index mutated in-place to the
new index before an enclosing struct.new or struct.set ran
ChildLocalizer or getResultOfFirst. When EffectAnalyzer subsequently
inspected those child instructions, readsStruct accessed
type.getStruct().fields[index] using the old HeapType and the new
index, causing an out-of-bounds assertion failure (__n < this->size())
or reading the wrong field's mutability.

Fix this by splitting instruction updating into two passes executed in a
single nested PassRunner invocation (FieldRemover followed by
IndexUpdater), along with sequential runOnModuleCode runs.
FieldRemover removes/reorders struct.new operands and replaces
removed struct.sets while all struct instructions still have their old
field indices matching their old HeapTypes, and IndexUpdater then
updates the field indices on all remaining struct instructions.