This migrates from 2-bit swizzle codes per lane to 4-bit, which reads more fluently in hex.
The order has been reversed so that the most significant nibble (leftmost on little-endian CPUs) represents the first output lane.

Fixed up Swizzle, Mask, and ShuffleLowHigh

Bug: b/145746360
Change-Id: I132bd757801a5ca543ef2b9b0ed34d0527db5bfa
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39049
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: David Turner <digit@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>