RMC can't handle u128 constants

[danielsn]

I tried this code:

let x : u128 = u128::MAX;

using the following command line invocation:

rmc main.rs

with RMC version:

I expected to see this happen: Nothing interresting.

Instead, this happened: RMC crashed, unable to handle the large constant.

thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: TryFromIntError(())', compiler/rustc_codegen_llvm/src/gotoc/cbmc/goto_program/expr.rs:462:30
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.54.0-dev running on x86_64-apple-darwin

note: compiler flags: -Z codegen-backend=gotoc

query stack during panic:
end of query stack