The Subzero JIT only supports constants in the second operand of
arithmetic operations, not the first. It assumes constant folding
already took place (true for NaCl which takes LLVM IR as input).
Reactor has constant folding as part of the static C++ compilation,
but the Optimizer may substitute the first operand for a constant
(i.e. constant propagation).

Addressing it in the Optimizer by not performing the substitution
is not trivial because we'd have to check each use. And it would
cost run-time performance. Ideally we'd have a pass for
legalization/optimization which performs constant folding.

For now, avoid hitting 'unreachable' code in Subzero by multiplying
constants at the Reactor level.

Fixes regression caused by https://swiftshader-review.googlesource.com/22910

Bug chromium:904265
Bug swiftshader:14

Change-Id: Ifdc72ac997ad5d71c1f7006259f54f3d715cb613
Reviewed-on: https://swiftshader-review.googlesource.com/c/22930Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>