Quiet SNaN inputs in Half.toFloat()

This CL fixes Half.toFloat() in handling SNaN inputs,
to make sure the Java implementation has same semantics
as the hardware implementations (Arm and Intel).

1. Armv8.2 implementation (FCVT):
  // tested as ART intrinsic on Pixel3
  __ Fmov(h31, w1);  // input: w1 register
  __ Fcvt(s0, h31);  // output: s0 register

2. X86 implementation (VCVTPH2PS):
  // clang -mf16c test.c
  float x86_toFloat(short h) {
    __v8hi v = {h, 0, 0, 0, 0, 0, 0, 0};
    __v4sf r = __builtin_ia32_vcvtph2ps(v);
    return r[0];
  }

3. Java implementation (software):
  android.util.Half.toFloat(short);

Test: Exhaustive testing of 0x0..0xFFFF input bits on above three
      implementations and compare output values.

Change-Id: Iff137858379bf43e59cde94c9d19c2054a3d4f93