Increases minimum CAS size from 16 bit to 32 bit, for better SASS
codegen.
When atomics are emulated using atom.cas.b16, the SASS generated
includes 2 (nested) emulation loops. When emulated using an atom.cas.b32
loop, the SASS too has a single emulation loop. Using 32 bit CAS thus
results in better codegen.
892a804d93