xen/riscv: drop INSN_{16,32}BIT_MASK and report unknown instruction lengths

Both constants were inherited from OpenSBI and their names are misleading.
The length of an instruction is encoded in its low bits: bits[1:0] != 11
means 16 bits, otherwise bits[4:2] != 111 means 32 bits, and anything else
is 48 bits or wider. 0x3 and 0x1c are the patterns compared against in
these two tests, not masks selecting a 16-bit or a 32-bit instruction. As
each constant is needed in one place only, once INSN_IS_16BIT() and
INSN_IS_32BIT() encapsulate the tests, drop them and spell the values out
in those two macros instead.

While at it, make INSN_LEN() cover the case both macros already knew about:
an encoding of 48 bits or wider now yields 0 rather than being silently
taken for a 32-bit instruction. No ratified extension defines an instruction
of such a length, so rather than open-coding a decoder for encodings which
cannot legitimately occur, leave it to the caller to treat the 0 as an
illegal instruction.

No functional change: the only present caller applies INSN_LEN() to a bug
frame, i.e. to an instruction emitted by Xen itself, which is either 16 or
32 bits wide.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Baptiste Le Duc <baptiste.le-duc@vates.tech>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/include/asm/riscv_encoding.h

@@ -857,16 +857,18 @@
 #error "Unexpected __riscv_xlen"
 #endif
 
-#define INSN_16BIT_MASK			0x3
-#define INSN_32BIT_MASK			0x1c
+#define INSN_IS_16BIT(insn)	(((insn) & 3) != 3)
+#define INSN_IS_32BIT(insn)	(!INSN_IS_16BIT(insn) && ((insn) & 0x1c) != 0x1c)
 
-#define INSN_IS_16BIT(insn)		\
-	(((insn) & INSN_16BIT_MASK) != INSN_16BIT_MASK)
-#define INSN_IS_32BIT(insn)		\
-	(((insn) & INSN_16BIT_MASK) == INSN_16BIT_MASK && \
-	 ((insn) & INSN_32BIT_MASK) != INSN_32BIT_MASK)
-
-#define INSN_LEN(insn)			(INSN_IS_16BIT(insn) ? 2 : 4)
+/*
+ * Length in bytes of the instruction whose first parcel is @insn: 2 or 4, or
+ * 0 when the encoding is 48 bits or wider. No ratified extension defines an
+ * instruction of such a length, so rather than open-coding a decoder for
+ * encodings which cannot legitimately occur, leave it to the caller to treat
+ * the 0 as an illegal instruction.
+ */
+#define INSN_LEN(insn) \
+	(INSN_IS_16BIT(insn) ? 2 : (INSN_IS_32BIT(insn) ? 4 : 0))
 
 #if __riscv_xlen == 64
 #define LOG_REGBYTES			3