cmd/compile/internal/ssagen: use an alias for math/bits.Len

cmd/compile/internal/ssagen: use an alias for math/bits.Len

Rather than using a specific intrinsic for math/bits.Len, use a pair of
aliases instead. This requires less code and automatically adapts when
platforms have a math/bits.Len32 or math/bits.Len64 intrinsic.

Change-Id: I28b300172daaee26ef82a7530d9e96123663f541
Reviewed-on: https://go-review.googlesource.com/c/go/+/656995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>