[llvm] [IR] Specify that overflow is poison, not UB in {s,u}{mul,div}.fix (PR #141748)

[Luke Lau via llvm-commits]

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@@ -19135,8 +19137,10 @@ If the result value cannot be precisely represented in the given scale, the
 value is rounded up or down to the closest representable value. The rounding
 direction is unspecified.
 
-It is undefined behavior if the result value does not fit within the range of
-the fixed point type, or if the second argument is zero.
+If the result value does not fit within the range of the fixed point type, the
+result is poison.
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lukel97 wrote:

Can it overflow if you divide by a fractional value? e.g. i8 udiv.fix(i8 255, i8 1, i8 1) = 127.5 / 0.5 = 255, which > 127.5?

https://github.com/llvm/llvm-project/pull/141748