[Beignet] constant expression bug Re: pow(n), erf(c), tgamma give wrong results

[Rebecca N. Palmer]

>>> And it triggers some other bugs related to constant expression
>>> handling when run all unit test cases.
>> I didn't see that when I ran the tests in 0.9.3 on IvyBridge M GT2
>> (only builtin_pow and builtin_tgamma failed, both due to the
>> absolute error checking you've already fixed) but your patch says
>> the problem does exist in 0.9.x
Done some more tests: it does fail in plain 0.9-head (in 
compiler_constant_expr) with
ASSERTION FAILED: 0 && "unsupported ce opcode.\n"
   at file 
/home/rnpalmer/Debian/builds/stackbuild/beignet/backend/src/llvm/llvm_gen_backend.cpp, 
function gbe::ir::ImmediateIndex 
gbe::GenWriter::processConstantImmIndex(llvm::Constant*, int32_t), line 945
but not in Debian 0.9, probably because GBE_DEBUG is off in Debian's 
build so GBE_ASSERT does nothing.