[AArch64][SME] Remove get.pstatesm intrinsic.

This intrinsic can be removed in favour of using a call to
__arm_sme_state() directly and testing the LSB of X0.

In IR that would look like:

   %pstate = call aarch64_sme_preservemost_from_x2 {i64, i64} @__arm_sme_state()
   %pstate.x0 = extractvalue {i64, i64} %pstate, 0
   %pstate.sm = and i64 %pstate.x0, 1