Auto merge of #136035 - SpecificProtagonist:miri-zeroed-alloc, r=oli-obk

miri: optimize zeroed alloc

When allocating zero-initialized memory in MIR interpretation, rustc allocates zeroed memory, marks it as initialized and then re-zeroes it. Remove the last step.

I don't expect this to have much of an effect on performance normally, but in my case in which I'm creating a large allocation via mmap it gets in the way.

Author: bors