[IslAst] Do not assert in case of empty min/max alias locations

In certain situations, the context in the isl_ast_build could result for the
min/max locations of our alias sets to become empty, which would cause an
internal error in isl, which is then unable to derive a value for these
expressions. Check these conditions before code generating expressions and
instead assume that alias check succeeded. This is valid, as the corresponding
memory accesses will not be executed under any valid context.

This fixed llvm.org/PR34432. Thanks to Qirun Zhang for reporting.

llvm-svn: 312455

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