[MC] Minor cleanup to MCFixup::Kind handling. NFC.

Prefer `MCFixupKind` where possible and add getTargetKind() to
convert to `unsigned` when needed rather than scattering cast
operators around the place.

Differential Revision: https://reviews.llvm.org/D59890

llvm-svn: 369720
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
index 1122f27..cab2bbc 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
@@ -50,7 +50,7 @@
                                             bool IsPCRel) const {
   const MCExpr *Expr = Fixup.getValue();
   // Determine the type of the relocation
-  unsigned Kind = Fixup.getKind();
+  unsigned Kind = Fixup.getTargetKind();
   if (IsPCRel) {
     switch (Kind) {
     default: