Fix ResolverSyntheticInstanceof incorrectly converting non-boolean ModifyExpressionValue to ModifyInstanceofValue

[SavageVictor]

Problem

When a @ModifyExpressionValue mixin targets a method call whose result is used in a nearby instanceof check, ResolverSyntheticInstanceof (via the skipInsnComparison path) would unconditionally convert the annotation to @ModifyInstanceofValue.

This is only correct when the handler returns boolean (i.e. it truly modifies the boolean result of the instanceof check). If the handler returns a non-boolean type — because it modifies the value fed into the instanceof check — the conversion produces an invalid method signature like (LBlock;)LBlock; where (Z)Z is required, crashing at mixin apply time with:

InvalidInjectionException: @ModifyInstanceofValue ... has an invalid signature.
Found unexpected return type Block, expected boolean.
Handler signature: (LBlock;)LBlock; Expected signature: (Z)Z

Reproduction

BigGlobe's Biome_DontFreezeRiverWater mixin uses @ModifyExpressionValue with handler signature (Block)Block targeting BlockState.getBlock(), whose result is then compared via instanceof FluidBlock. The converter sees the nearby instanceof and incorrectly rewrites the mixin as @ModifyInstanceofValue, crashing the game on bootstrap.

Fix

Before converting, check the handler's return type via Type.getReturnType(context.methodNode().desc). If it is not boolean, skip the conversion and return pass().

Testing

Minecraft: 1.21.1
NeoForge: 21.1.172

Mod	Version
Connector	2.0.0-beta.14+1.21.1
ConnectorExtras	1.12.1+1.21.1
Forgified Fabric API	0.116.7+2.2.4+1.21.1
Big Globe	5.3.1-MC1.21.1
Iris (NeoForge)	1.8.12+mc1.21.1
Sodium (NeoForge)	0.6.13+mc1.21.1

With the fix applied, Biome_DontFreezeRiverWater is correctly left as @ModifyExpressionValue and the game boots successfully.