Simplify the getConstant code during loop analysis.
Simplify the getConstant code during loop analysis.

This function took a unique_ptr<Expression>& but it was sufficient to
take just an Expression&. This doesn't change anything in this CL, but
I have been experimenting with ownership changes of VarDecl expressions
(due to fuzzer findings) and not requiring a unique_ptr here has been
helpful.

Change-Id: Ic6851e8171779b77c6e5caf45903f2d99dd012fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/389797
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

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