target/arm: Hoist arm_is_el2_enabled check in sve_exception_el

target/arm: Hoist arm_is_el2_enabled check in sve_exception_el

This check is buried within arm_hcr_el2_eff(), but since we
have to have the explicit check for CPTR_EL2.TZ, we might as
well just check it once at the beginning of the block.

Once this is done, we can test HCR_EL2.{E2H,TGE} directly,
rather than going through arm_hcr_el2_eff().

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220607203306.657998-9-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>