Fix ARM vcvt encoding when the number of fractional bits is zero.

The instruction to convert between floating point and fixed point representations
takes an immediate operand for the number of fractional bits of the fixed point
value. ARMARM specifies that when that number of bits is zero, the assembler
should encode floating point/integer conversion instructions. 

This patch adds the necessary instruction aliases to achieve this behaviour.

llvm-svn: 189009