Rollup merge of #56233 - kenta7777:kenta7777#49937, r=oli-obk
Rollup merge of #56233 - kenta7777:kenta7777#49937, r=oli-obk

Miri and miri-related code contains repetitions of `(n << amt) >> amt`

I reduced some code repetitions contains `(n << amt) >> amt`.
This pull request is related to #49937.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust