loongarch gas resolving constant expressions

The test added in commit 4fe96ddaf614 results in an asan complaint:
loongarch-parse.y:225:16: runtime error: left shift of negative value -1
To avoid the complaint, perform left shifts as unsigned (which gives
the same result on 2's complement machines).  Do the same for
addition, subtraction and multiplication.  Furthermore, warn on
divide/modulus by zero.