constant switch test bug in x86Translate

[Stephen Weeks]

In attempting to find a nasty G1 bug, I turned off a bunch of passes in the CPS
simplifier.  This tickled a silly bug in x86Translate.  It fails on constant
switch tests.  For example, mlton died in a self compile with the error
	mlton:toX86Blocks: Switch
on the following switch statement
	Switch (3, [(0, L_4175), (1, L_4176), (2, L_4177), (3, L_4178)], None)

Is it hard to make the codegen handle this?