[AArch64][SVE] Asm: Improve diagnostics for gather loads.

This patch extends the 'isSVEVectorRegWithShiftExtend' function to 
improve diagnostics for SVE's gather load (scalar + vector) addressing 
modes. Instead of always suggesting the 'unscaled' addressing mode, 
the use of DiagnosticPredicate enables a more specific error message
in the context where the scaling is incorrect. For example:

  ld1h z0.d, p0/z, [x0, z0.d, lsl #2]
                                   ^ 
           shift amount should be '1'

Instead of suggesting the packed, unscaled addressing mode:
  expected 'z[0..31].d, (uxtw|sxtw)'

the assembler now suggests using the proper scaling:
  expected 'z[0..31].d, (lsl|uxtw|sxtw) #1'

Reviewers: fhahn, rengolin, samparker, SjoerdMeijer, javed.absar

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D46124

llvm-svn: 331162