cmd/compile: repair name propagation into aggregate parts
For structs, slices, strings, interfaces, etc, propagation of
names to their components (e.g., complex.real, complex.imag)
is fragile (depends on phase ordering) and not done right
for the "dec" pass.
The dec pass is subsumed into decomposeBuiltin,
and then names are pushed into the args of all
OpFooMake opcodes.
compile/ssa/debug_test.go was fixed to pay attention to
variable values, and the reference files include checks
for the fixes in this CL (which make debugging better).