nir/print: describe intrinsic sources

Hunting down what individual intrinsic sources mean gets old pretty fast. Let's print this information where possible. Also print indices in a bit more compact way.

One example, before:

intrinsic store_output (ssa_10, ssa_69) (8, 15, 0, 160, 132) /* base=8 */ /* wrmask=xyzw */ /* component=0 */ /* src_type=float32 */ /* location=4 slots=1 */ /* dp_FragColor */

after:

intrinsic store_output (value=ssa_10, offset=ssa_69) (base=8, wrmask=xyzw /*15*/, component=0, src_type=float32 /*160*/, io location=4 slots=1 /*132*/) /* dp_FragColor */

Source names are added only to intrinsics which had this information in the preceding comment.

Edit: indices changes were merged via !14222 (merged).