bring back lowp bilerp_clamp_8888
bring back lowp bilerp_clamp_8888

PS1-3 are the literal code I removed a few months ago.

PS4 adds asserts to make sure we don't underflow.
PS5 adds rounding when scaling off bias, reducing diff slightly.

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: Ice573395a1680e1c3b55a8fe3e4d034f15adde1b
Reviewed-on: https://skia-review.googlesource.com/c/196620
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

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