condense stack usage
condense stack usage

Only use a stack slot when spilled being careful to track hoised
slot data between vector and scalar code generation.

TODO:
Come up with a strategy to change the constant on the initial
subtrack from SP to reduce size.

Change-Id: Icf38ad33c7026dac065d51547db827b1516f2504
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285107
Commit-Queue: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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