[PowerPC][GISel]support for float point and integer convertion

Add support for fptosi,fptoui,sitofp,uitofp

For now only handle 64 bit integer to make it does not depend on
any other patches. 32 bit integer needs handling for G_SEXT/G_ZEXT.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D139174