Remove the API for creating ConstantExprs with the nsw, nuw, inbounds,

and exact flags. Because ConstantExprs are uniqued, creating an
expression with this flag causes all expressions with the same operands
to have the same flag, which may not be safe. Add, sub, mul, and sdiv
ConstantExprs are usually folded anyway, so the main interesting flag
here is inbounds, and the constant folder already knows how to set the
inbounds flag automatically in most cases, so there isn't an urgent need
for the API support.

This can be reconsidered in the future, but for now just removing these
API bits eliminates a source of potential trouble with little downside.

llvm-svn: 80959