NFC: improve ARM64 isFPImmLegal debug print

Forking this change from D51706. This just made it easier to understand llc
output with -debug.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341504 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed