[RISCV] Prepare for the use of variable-sized register classes

While parameterising by XLen, also take the opportunity to clean up the 
formatting of the RISCV .td files.

This commit unifies the in-tree code with my patchset at 
<https://github.com/lowrisc/riscv-llvm>.

llvm-svn: 316159
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
index f821215..f94c37a 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
@@ -159,7 +159,7 @@
              cast<MCSymbolRefExpr>(Expr)->getKind() == MCSymbolRefExpr::VK_None) {
     if (Desc.getOpcode() == RISCV::JAL) {
       FixupKind = RISCV::fixup_riscv_jal;
-    } else if (MIFrm == RISCVII::InstFormatSB) {
+    } else if (MIFrm == RISCVII::InstFormatB) {
       FixupKind = RISCV::fixup_riscv_branch;
     }
   }