Cleans up the implementation of trapping/saturating operations to
use existing models of types, instead of defining new ones. That is,
it uses MachineType to represent the kind of integer/float value the
conversion is operating on.

It also removes the need to define NumericImplementation by using
the actual opcodes to determine if the instruction is trapping or
saturating.

Finally, it now defines a single method to implement the truncating
(that doesn't use a ccall) conversions. The previous code had two
methods, one for I32 and one for I64.

Bug: v8:7226
Change-Id: Ia869416c8bd1abe76d0583c28a3105102d130b46
Reviewed-on: https://chromium-review.googlesource.com/900558
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51129}