Implement normalize intrinsic in RP codegen.

We implement normalize as `x / length(x)`, which matches the SkVM
implementation.

A zero-length vector will return nan; the GLSL docs don't specify
what should happen in this case: http://screen/3TsNoijNDxN5xQZ

Change-Id: I4c57e54fdbc0d986bb8d1adb52ec0889f9d9846b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/657816
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>