[llvm] [GlobalISel] Support saturated truncate (PR #150219)

[David Green via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
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davemgreen wrote:

Could we use the existing test coverage in for example qmovn.ll and maybe fpclamptosat_vec.ll?

https://github.com/llvm/llvm-project/pull/150219