gccrs: Fix ICE when checking shift's which are behind array refs

gccrs: Fix ICE when checking shift's which are behind array refs

I copied a bad form of this check from the c front-end this updates it
to ensure the rhs is an INTEGER_CST and the lhs needs checked in the first
place.

Fixes Rust-GCC#3664

gcc/rust/ChangeLog:

	* rust-gcc.cc (arithmetic_or_logical_expression): Ensure this is an integer

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3664.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>