No Math numerical negation block.

[matthewtebbs]

Issue #4109 comments on a round trip issue when decompiling from an expression using the '-=' subtraction assigment operator.

This issue suggests adding a negation block which would replace the '0 - x' hack in this decompilation.

For example,

material.roughness -= myroughness

would decompile to

but with a '-x' negation block replacing the '0 - x' expression.