Remove allOffMaskIsSafe from Target

The intent of this was to indicate whether it was safe to run code
with an 'all of' mask on the given target (and then sometimes be
more flexible about e.g. running both true and false blocks of if
statements, etc.)

The problem is that even if the architecture has full native mask support,
it's still not safe to run 'uniform' memory operations with the mask all
off.  Even more tricky, we sometimes transform masked varying memory operations
to uniform ones during optimization (e.g. gather->load and broadcast).

This fixes a number of the tests/switch-* tests that were failing on the
generic targets due to this issue.

ispc.h

@@ -239,12 +239,6 @@ struct Target {
         natively. */
     bool maskingIsFree;
 
-    /** Is it safe to run code with the mask all if: e.g. on SSE, the fast
-        gather trick assumes that at least one program instance is running
-        (so that it can safely assume that the array base pointer is
-        valid). */
-    bool allOffMaskIsSafe;
-
     /** How many bits are used to store each element of the mask: e.g. this
         is 32 on SSE/AVX, since that matches the HW better, but it's 1 for
         the generic target. */