[SVE][LoopVectorize][NFC] Tidy up some tests

Whilst writing a patch to add extra tail-folding RUN lines to
existing tests I noticed a few areas where they can be
cleaned up a little:

1. scalable-reductions.ll: fmin_fast does not mark fcmp as fast.
2. sve-inductions-unusual-types.ll: remove direct references to
   SSA variable names.
3. sve-strict-fadd-cost.ll: don't force vector width so we see
   costs for different VFs in one go. This will be important for
   the follow-on patch.
4. sve-vector-reverse.ll,vector-reverse-mask4.ll: add noalias
   keyword to simplify IR.
4. sve-widen-gep.ll,sve-widen-phi.ll: regenerate using script.

These changes will make the subsequent patch adding RUN lines much
easier to review!

Differential Revision: https://reviews.llvm.org/D132219