target/arm: Rename SME BFCVT patterns to BFCVT_hs

The existing pattern is BFCVT (single-precision to BFloat16).
In preparation for introducing more insns of the same name,
append the operand sizes.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20260609192110.752384-10-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>