[AArch64] Perform first active true vector combine

Materialize : i1 = extract_vector_elt t37, Constant:i64<0>
   ... into: "ptrue p, all" + PTEST
Test bit of lane 0 can use P register directly, and the instruction “pture all”
is loop invariant, which will beneficial to SVE after hoisting out the loop.

Reviewed By: david-arm, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D120891