Offer branchless conditional (CMOV) primop

I'd like to see something similar to C's ?:, like

ifI#, boolI# :: Int# -> Int# -> Int# -> Int#

We could then write things like

ifI# (x <# y) 12# (x +# 17#)
boolI# potato cannon (y ># 2)

When the value of the test is difficult to predict, this would be better than the usual branching if or case.