[llvm] [MIPS]Remove unnecessary SLL instructions on MIPS64el (PR #109386)

[YunQiang Su via llvm-commits]

wzssyqa wrote:

Please read: **https://llvm.org/docs/LangRef.html#id2158**
The IR instruction `trunc` has a IMM argument. It will be quilte different if the IMM is different.

https://github.com/llvm/llvm-project/pull/109386