making D.1751_2 no longer single-use and thus no longer triggering combine.
Indeed looks related to 50176.
But while we certainly can teach forwprop to only consider single-use
chains (to never possibly cause this issue) it isn't a good solution.
In fact for properly optimizing this we need to know whether cheap
sub-reg like accesses are possible (combining (int) (uint8_t) (int64_t)
code[rdx_1] to simply extending the lower part of (int64_t) code[rdx_1]
without explicit truncation). This seems more fit for an RTL optimization
pass than for a tree pass if consider the forwprop "optimization" be done
in source like