[llvm] [RISCV][ISel] Remove redundant min/max in saturating truncation (PR #75145)

[Craig Topper via llvm-commits]

topperc wrote:

> > How can you use just vnsrl? vnsrl discards the upper bits without any max or min.
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> Yes, you are right. 
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> But if the range created by a pair of max/min operation is precisely the range of the truncation destination, 
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> then the pair could be removed.
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> In the above case, `[-128, 127]` is the value range of u8. 
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> Since we truncate the u16 vector into a u8 vector, removing the max/min won't change the final result, AFAIU. 

Let's imagine the input is -32767(0x8001) in i16. The smax with -128 should give -128(0xff80). Then it will be truncated to 0x80 which is -128 in i8.

A vnsrl by itself will drop the upper bits and give 0x01. This is incorrect.

https://github.com/llvm/llvm-project/pull/75145