Combine fVariableMap and fReturnValueMap into one map.
Combine fVariableMap and fReturnValueMap into one map.

Both maps are tracking the same information (slot value associated with
a given SkSL IR construct) so it probably reduces overhead to combine
them into one.

Also, replaced unordered_map with SkTHashMap since Skia hash maps tend
to be much more efficient in general.

Change-Id: I80f8fc214423685c888b18ead6d4148e32a7ccb6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/515116
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia