[AsmPrinter] Print FP constant in hexadecimal form instead

Printing floating point number in decimal is inconvenient for humans.
Verbose asm output will print out floating point values in comments, it
helps.

But in lots of cases, users still need additional work to covert the
decimal back to hex or binary to check the bit patterns,
especially when there are small precision difference.

Hexadecimal form is one of the supported form in LLVM IR, and easier for
debugging.

This patch try to print all FP constant in hex form instead.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D73566