[RISCV] Allow RISCVMCExpr folding in the absence of linker relaxaxation

In the AArch64 target, `bar: add x0, x0, :lo12:foo-bar; foo:`
can be folded.

Remove the restriction (https://reviews.llvm.org/D104473), which became
unnecessary after https://reviews.llvm.org/D155357

Author: MaskRay

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp

@@ -91,10 +91,7 @@ const MCFixup *RISCVMCExpr::getPCRelHiFixup(const MCFragment **DFOut) const {
 bool RISCVMCExpr::evaluateAsRelocatableImpl(MCValue &Res,
                                             const MCAssembler *Asm,
                                             const MCFixup *Fixup) const {
-  // Explicitly drop the layout and assembler to prevent any symbolic folding in
-  // the expression handling.  This is required to preserve symbolic difference
-  // expressions to emit the paired relocations.
-  if (!getSubExpr()->evaluateAsRelocatable(Res, nullptr, nullptr))
+  if (!getSubExpr()->evaluateAsRelocatable(Res, Asm, Fixup))
     return false;
 
   Res =

llvm/test/MC/RISCV/hilo-constaddr-expr.s

@@ -1,10 +1,15 @@
 # RUN: not llvm-mc -filetype=obj -triple=riscv32 -mattr=+relax %s -o /dev/null 2>&1 | FileCheck %s
-# RUN: not llvm-mc -filetype=obj -triple=riscv32 -mattr=-relax %s -o /dev/null 2>&1 | FileCheck %s
+# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=-relax %s | llvm-objdump -d - | FileCheck %s --check-prefix=NORELAX
 
 # Check the assembler rejects hi and lo expressions with constant expressions
 # involving labels when diff expressions are emitted as relocation pairs.
 # Test case derived from test/MC/Mips/hilo-addressing.s
 
+# NORELAX:      lui t0, 0x0
+# NORELAX-NEXT: lw ra, 0x8(t0)
+# NORELAX:      lui t1, 0x0
+# NORELAX-NEXT: lw sp, -0x8(t1)
+
 tmp1:
 tmp2:
 # CHECK: :[[#@LINE+1]]:[[#]]: error: expected relocatable expression