AMDGPU: Assume all amdhsa kernarg passed implicit arguments by default

Previously we would require adding an attribute to kernels to enable
the inputs passed in the kernarg segment, accessed by
llvm.amdgcn.implicitarg.ptr. This violates the principle of being
correct by default. Some OpenMP testcases were broken recently since
it wasn't correctly setting this attribute, and no known frontends are
setting this to anything other than the maximum.

Most of the test changes are from load widening of argument loads
since there now more implied dereferenceable bytes.