Rename IntLiteral to IntValue. (#4475)

This instruction represents integer values, whether they come from
literals or calculations, so it the old name is inaccurate. I also plan
to rename `BigInt` to `IntLiteral` based on recent discussion and this
change aims to avoid confusion stemming from the same name being used
for two different things.

I'm not renaming `FloatLiteral` because recent discussion suggests we
may want distinct `FloatLiteral` versus `FloatValue` representations in
SemIR.

toolchain/lower/constant.cpp

@@ -204,7 +204,7 @@ static auto EmitAsConstant(ConstantContext& /*context*/,
   return nullptr;
 }
 
-static auto EmitAsConstant(ConstantContext& context, SemIR::IntLiteral inst)
+static auto EmitAsConstant(ConstantContext& context, SemIR::IntValue inst)
     -> llvm::Constant* {
   auto* type = context.GetType(inst.type_id);