`VPEVLBasedIVPHIRecipe` will lower to VPInstruction scalar phi and
generate scalar phi. This recipe will only occupy a scalar register just
like other phi recipes.
This patch fix the register usage for `VPEVLBasedIVPHIRecipe` from
vector
to scalar which is close to generated vector IR.
https://godbolt.org/z/6Mzd6W6ha shows that no register spills when
choosing `<vscale x 16>`.
Note that this test is basically copied from AArch64.
d611a9ca15