fix: prevent DCE from removing throwing bigint unary minus

[Kenaz123]

Summary

Fix DCE incorrectly removing throwing unary minus on BigInt values.

Before this change, UnaryOperatorInst::getSideEffectImpl() treated any unary operation on a primitive operand as idempotent:

SideEffect UnaryOperatorInst::getSideEffectImpl() const {
  if (getSingleOperand()->getType().isPrimitive()) {
    return SideEffect{}.setIdempotent();
  }
  ...
}

That is wrong for UnaryMinusInst when the operand can be a BigInt. Negating a negative max-size BigInt may need one extra digit in the result, which throws RangeError: Maximum BigInt size exceeded. Because the instruction was marked
idempotent, DCE could delete an unused -bigint and silently suppress the exception under -O.

This change fixes that by marking unary minus as throwing when its operand canBeBigInt():

if (getKind() == ValueKind::UnaryMinusInstKind &&
    getSingleOperand()->getType().canBeBigInt()) {
  return SideEffect{}.setThrow();
}

poc.js

Original PoC reproduction:

./build-static_h/bin/hermes -O ./poc.js
./build-static_h/bin/hermes -O0 ./poc.js

Original output before the fix:

$ ./build-static_h/bin/hermes -O poc.js
Bug: RangeError was suppressed by DCE

$ ./build-static_h/bin/hermes -O0 poc.js
Uncaught RangeError: Maximum BigInt size exceeded

Test Plan

cmake -S ./hermes -B ./hermes/build-static_h -G Ninja -DCMAKE_BUILD_TYPE=Debug
cmake --build ./hermes/build-static_h --target hermes hermesc shermes -j4

./hermes/build-static_h/bin/hermes -O0 \
  ./hermes/test/shermes/regress-bigint-unary-minus-dce.js

./hermes/build-static_h/bin/hermes -O \
  ./hermes/test/shermes/regress-bigint-unary-minus-dce.js

./hermes/build-static_h/bin/hermesc -dump-ir -O \
  ./hermes/test/Optimizer/regress-bigint-unary-minus-dce.js

Observed results:

$ hermes -O0 test/shermes/regress-bigint-unary-minus-dce.js
RangeError: Maximum BigInt size exceeded

$ hermes -O test/shermes/regress-bigint-unary-minus-dce.js
RangeError: Maximum BigInt size exceeded

$ hermesc -dump-ir -O test/Optimizer/regress-bigint-unary-minus-dce.js
...
%2 = UnaryMinusInst (:bigint) 1: bigint
%4 = UnaryMinusInst (:number|bigint) %3: any
%5 = UnaryMinusInst (:number|bigint) %4: number|bigint
...

Thanks for looking into this and I appreciate any feedback!