Auto merge of #38989 - arielb1:constant-mir-overflow2, r=eddyb
fix function arguments in constant promotion
we can't create the target block until *after* we promote the arguments - otherwise the arguments will be promoted into the target block. oops.
Fixes #38985.
This is a regression introduced in the beta-nominated #38833, so beta-nominating this one too (sorry @brson).
r? @eddyb