Fix GT_MULHI register requirements

Fix GT_MULHI register requirements

We only care about the upper 32 bits of the 64 bit result and those are
in EDX. It doesn't make sense to set the destination candidate to EAX,
most of the time this will result in a useless `mov eax, edx` being
generated.

Also, genCodeForMulHi needs to check if the implicit operand is in EAX
rather than targetReg.

Commit migrated from https://github.com/dotnet/coreclr/commit/0822b57867aaae098239022e7ebaef745588b8a4