[llvm] [PowerPC] Fix assert exposed by PR 95931 in LowerBITCAST (PR #108062)

[Amy Kwan via llvm-commits]

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@@ -86,3 +86,25 @@ entry:
   ret i64 %1
 }
 
+define <4 x i32> @test(i512 %a) {
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amy-kwan wrote:

nit: Can we maybe be a bit more descriptive for the test title? Not sure if something like `truncBitcast` makes sense, or something like that?

https://github.com/llvm/llvm-project/pull/108062