MFV: less v458.

MFC after:	2 weeks
Pull in r181286 from upstream llvm trunk:

  LoopVectorize: getConsecutiveVector must respect signed arithmetic

  We were passing an i32 to ConstantInt::get where an i64 was needed and we must
  also pass the sign if we pass negatives numbers. The start index passed to
  getConsecutiveVector must also be signed.

  Should fix PR15882.

This should fix Firefox crashes some people have been reporting, when it
is compiled with -O3.
Bandaid for compiling with gcc, which happens to be the default compiler
for a number of platforms still.