mips: Floating point convert bug fix

mips: Floating point convert bug fix

In mips architecture, floating point convert instructions use the
FloatConvertOp format defined in src/arch/mips/isa/formats/fp.isa. The type
of the operands in the ISA description file (_sw for signed word, or _sf for
signed float, etc.) is  used to create a type for the operand in C++. Then the
operand is converted using the fpConvert() function in src/arch/mips/utility.cc.

If we are converting from a word to a float, and we want to convert 0xffffffff,
we expect -1 to be passed into fpConvert(). Instead, we see MAX_INT passed in.
Then fpConvert() converts _val_ to MAX_INT in single-precision floating point,
and we get the wrong value.

To fix it, the signs of the convert operands are being changed from unsigned to
signed in the MIPS ISA description.

Then, the FloatConvertOp format is being changed to insert a int32_t into the
C++ code instead of a uint32_t.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>