[clang-fuzzer] Made loop_proto more "vectorizable".

Edited loop_proto and its converter to make more "vectorizable" code
according to kcc's comment in D47666
  - Removed all while loops
  - Can only index into array with induction variable

Patch By: emmettneyman

Differential Revision: https://reviews.llvm.org/D47920

llvm-svn: 334252