[Decompiler] Fix coloring moves

[water111]

The expression propagator doesn't attempt to run on reg-to-reg consuming moves and instead lets the FormStack deal with this. The FormStack already did the right thing for pop_reg, with the recursive call to pop_reg, but it did not totally handle two cases, which were added:

1. The case where there is a coloring move result to get the return value. Now we can loop up the expression stack following consecutive coloring sets.
2. Just eliminating these.
   TODO - there is an issue with this:

(set! x (if a (set! y z)))

will probably be decompiled as (set! x (if a z)) and eliminate the set! y.

[coveralls]

Pull Request Test Coverage Report for Build 597772893

• 56 of 73 (76.71%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 68.827%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
decompiler/IR2/FormStack.cpp	37	54	68.52%

Files with Coverage Reduction	New Missed Lines	%
decompiler/IR2/Form.h	1	94.92%
decompiler/IR2/FormStack.cpp	1	77.23%

Totals
Change from base Build 589473161:	0.01%
Covered Lines:	30209
Relevant Lines:	43891

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💛 - Coveralls