[SelectionDAG] Support promotion of the FPOWI integer operand

[SelectionDAG] Support promotion of the FPOWI integer operand

For targets where i32 is not a legal type (e.g. 64-bit RISC-V),
LegalizeIntegerTypes must promote the integer operand of ISD::FPOWI. As this
is a signed value, this should be sign-extended.

This patch enables all tests in test/CodeGen/RISCVfloat-intrinsics.ll for
RV64, as prior to this patch that file couldn't be compiled for RV64 due to an
assertion when performing codegen for fpowi.

Differential Revision: https://reviews.llvm.org/D54574

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352832 91177308-0d34-0410-b5e6-96231b3b80d8