Optimize String concatenation setup when using primitive operands

The current default String concatenation strategy adds in MHs to calculate the expected String coder even for primitives that by definition won't mutate the coder value.

Avoiding calculating and folding in a new coder value and instead re-using the current coder value reduces the number of intermediate MHs generated for string concatenation expressions with one or more primitive arguments, while not affecting peak performance characteristics negatively.