R600: Allow ftrunc

v2: Add ftrunc->TRUNC pattern instead of replacing int_AMDGPU_trunc
v3: move ftrunc pattern next to TRUNC definition, it's available since R600

Patch By: Jan Vesely

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>