[mlir][MemRefToLLVM] Add fmin, fmax to AtomicRMW lowering

[krzysz00]

Summary

Add cases to the memref.atomicrmw lowering for floating-point min and
max, since LLVM supports these.

mlir/lib/Conversion/MemRefToLLVM/MemRefToLLVM.cpp

 matchSimpleAtomicOp(memref::AtomicRMWOp atomicOp) {
   switch (atomicOp.getKind()) {
   case arith::AtomicRMWKind::addf:
     return LLVM::AtomicBinOp::fadd;
   case arith::AtomicRMWKind::addi:
     return LLVM::AtomicBinOp::add;
   case arith::AtomicRMWKind::assign:
     return LLVM::AtomicBinOp::xchg;
+  case arith::AtomicRMWKind::maxf:
+    return LLVM::AtomicBinOp::fmax;
   case arith::AtomicRMWKind::maxs:
     return LLVM::AtomicBinOp::max;
   case arith::AtomicRMWKind::maxu:
     return LLVM::AtomicBinOp::umax;
+  case arith::AtomicRMWKind::minf:
+    return LLVM::AtomicBinOp::fmin;
   case arith::AtomicRMWKind::mins:
     return LLVM::AtomicBinOp::min;
   case arith::AtomicRMWKind::minu:
     return LLVM::AtomicBinOp::umin;
   case arith::AtomicRMWKind::ori:
     return LLVM::AtomicBinOp::_or;
   case arith::AtomicRMWKind::andi:
     return LLVM::AtomicBinOp::_and;