AsmParser: test .double NaN and .double inf

Summary: It looks like this support was added to match GNU AS, but only tests
.float and not .double. I asked RedHat folks to confirm that 0x7fffffffffffffff
was indeed the right value for NaN.

Same for infinity, but it only has positive / negative encodings.

Reviewers: scanon, rjmccall

Subscribers: jkorous, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D55531

llvm-svn: 349376