PR26437, PR26438 UBSAN: tc-cr16.c left shifts and overflows

Always use unsigned constants in expressions generating masks.  The
following trys mightily to avoid UB (but hits it anyway with bits=32
and 0x7fffffff << 1), and worse, for 32-bit int, 64-bit long, bits=32
doesn't generate 0xffffffff.
    max = ((((1 << (bits - 1)) - 1) << 1) | 1);
results in -1, or max = 0xffffffffffffffff.

This patch fixes that problem, a possible shift exponent of -1U,
removes some dead code, and makes general tidies.

	PR26437
	PR26438
	* config/tc-cr16.c: Include limits.h, formatting.
	(CR16_PRINT): Wrap params in parentheses.  Remove parens from uses
	throughout file.
	(getconstant): Handle zero nbits.
	(print_operand): Simplify handling of index regs.
	(check_range): Use int32_t variables.  Correct range checks.

gas/ChangeLog

@@ -1,3 +1,15 @@
+2020-08-30  Alan Modra  <amodra@gmail.com>
+
+	PR26437
+	PR26438
+	* config/tc-cr16.c: Include limits.h, formatting.
+	(CR16_PRINT): Wrap params in parentheses.  Remove parens from uses
+	throughout file.
+	(getconstant): Handle zero nbits.
+	(print_operand): Use unsigned variables.  Simplify handling of
+	index regs.
+	(check_range): Use int32_t variables.  Correct range checks.
+
 2020-08-29  Alan Modra  <amodra@gmail.com>
 
 	PR 26481