[AArch64][SVE] Asm: Add parsing/printing support for exact FP immediates.

Some instructions require of a limited set of FP immediates as operands,
for example '#0.5 or #1.0' for SVE's FADD instruction.

This patch adds support for parsing and printing such FP immediates as
exact values (e.g. #0.499999 is not accepted for #0.5).

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

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D47711

llvm-svn: 334826