This extends the lowering of ceil, floor, nearbyint, rint, round, roundeven and
trunc. They are all very similar, so can reuse the same legalization info.
selectIntrinsicTrunc and selectIntrinsicRound can be removed as they can be
selected via tablegen patterns, and G_INTRINSIC_ROUNDEVEN is marked as a gisel
equivalent of froundeven. Otherwise this reuses the existing code, filling it
out to handle more types.
Differential Revision: https://reviews.llvm.org/D157679
cf65afbf93