Add flag -fassume-phsa that is on by default. If -fno-assume-phsa
is given, these optimizations are disabled.

With this flag, gccbrig can generate GENERIC that assumes we are
targeting a phsa-runtime based implementation, which allows us
to expose the work-item context accesses to retrieve WI IDs etc.
which helps optimizers.

First optimization that takes advantage of this is to get rid of
the setworkitemid calls whenever we have non-inlined calls that
use IDs internally.

Other optimizations added in this commit:

- expand absoluteid to similar level of simplicity as workitemid.
At the moment absoluteid is the best indexing ID to end up with
WG vectorization.
- propagate ID variables closer to their uses. This is mainly
to avoid known useless casts, which confuse at least scalar
evolution analysis.
- use signed long long for storing IDs. Unsigned integers have
defined wraparound semantics, which confuse at least scalar
evolution analysis, leading to unvectorizable WI loops.
- also refactor some BRIG function generation helpers to brig_function.
- no point in having the wi-loop as a for-loop. It's really
a do...while and SCEV can analyze it just fine still.
- add consts to ptrs etc. in BRIG builtin defs.
Improves optimization opportunities.
- add qualifiers to generated function parameters.
Const and restrict on the hidden local/private pointers,
the arg buffer and the context pointer help some optimizations.

From-SVN: r259957