[AMDGPU] Convert rcp to rcp_iflag

If a source of rcp instruction is a result of any conversion from
an integer convert it into rcp_iflag instruction. No FP exception
can ever happen except division by zero if a single precision rcp
argument is a representation of an integral number.

Differential Revision: https://reviews.llvm.org/D48569

llvm-svn: 335742