[llvm] [DXIL] Add lowering for `reversebits` and `trunc` (PR #86909)

[S. Bharadwaj Yadavalli via llvm-commits]

================
@@ -285,13 +285,19 @@ def RSqrt : DXILOpMapping<25, unary, int_dx_rsqrt,
                          "Returns the reciprocal of the square root of the specified value."
                          "rsqrt(x) = 1 / sqrt(x).",
                          [llvm_halforfloat_ty, LLVMMatchType<0>]>;
+def Rbits : DXILOpMapping<30, unary, int_bitreverse,
+                         "Returns the specified value with its bits reversed.",
+                         [llvm_anyint_ty, LLVMMatchType<0>]>;
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bharadwajy wrote:

```suggestion
                         "Returns the specified value with its bits reversed.">;
```

Since the inherited types of `int_bitreverse` are valid for `Rbits`, overriding is not necessary,

Additionally, consider adding this record after `Trunc` to maintain ascending order of opcode value.

https://github.com/llvm/llvm-project/pull/86909