[llvm-stress] Enhance scalar type selection from command line.

llvm-stress command line options like -generate-x86-fp80 has been replaced with one list-like option -types. E.g. llvm-stress -types=x86_fp80,i100,i256,half. Default types (i1, i8, i16, i32, i64, float, double) are always added at the beginning of that list.

Reviewers: hfinkel

Differential Revision: http://reviews.llvm.org/D10667

llvm-svn: 241896