[RISCV] Fix incorrect mask of shuffle vector in the test. (NFC)

[tclin914]

The mask of shuffle vector should be <u, u, 4, 6, 8, 10, 12, 14>, not <u, u, 4, 6, *6, 10, 12, 14> for steps of 2.

And the mask of suffle vector with an undef initial element has been supported by #118509.

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 93b3cbadc2b624f5daf0ce000684c3c154d0aca4 7e2233beb76f0c34098737f867e0c9d531524f3b llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shufflevector-vnsrl.ll

The following files introduce new uses of undef:

• llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shufflevector-vnsrl.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.

[preames]

LGTM - thanks.