[AArch64][GlobalISel] Widen s2 and s4 G_IMPLICIT_DEF + G_FREEZE
.clampScalar(0, s1, 64) .widenScalarToNextPow2(0, 8)
If you have s2 or s4, then widenScalarToNextPow2 does nothing.
This changes the widenScalarToNextPow2 rule to use s8 as the minimum type
instead, allowing us to correctly widen s2 and s4.
This does not impact s1, since it's marked as legal already.
Differential Revision: https://reviews.llvm.org/D106413