use constants to generate less llvm-ir for raw_vec functions #77068
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as a continuation of #72013 to make RawVec::grow smaller I tried to use constants.
I can see that grow_amortized generate 13% less llvm-Ir and in total 3% less llvm-ir is generated for rustc_middle
but maybe the constant calculations is more time then is saved in llvm.
can I get a perf run?