NFC: improve ARM64 isFPImmLegal debug print
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@341504 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm