[AArch64][SVE] Asm: PTRUE and PTRUES instructions

Summary: These instructions initialize a predicate vector from a pattern/immediate.

Reviewers: fhahn, rengolin, evandro, mcrosier, t.p.northover, samparker, olista01

Reviewed By: samparker

Subscribers: aemerson, javed.absar, tschuett, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D41819

llvm-svn: 323124