Turn unreachable error into assert

Function valueFromStringRepr() throws an error on missing 0x prefix when
parsing a number string into a value. However, getWildcardRegex() already
ensures that only text with the 0x prefix will match and be parsed,
making that error throwing code dead code. This commit turn the code
into an assert and remove the unit tests exercising that test
accordingly.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D150797