Minor improvement to genpreds.cc

Minor improvement to genpreds.cc

This simple patch implements Richard Biener's suggestion in comment #6
of PR tree-optimization/52171 (from February 2013) that the insn-preds
code generated by genpreds can avoid using strncmp when matching constant
strings of length one.

The effect of this patch is best explained by the diff of insn-preds.cc:
<       if (!strncmp (str + 1, "g", 1))
---
>       if (str[1] == 'g')
3104c3104
<       if (!strncmp (str + 1, "m", 1))
---
>       if (str[1] == 'm')
3106c3106
<       if (!strncmp (str + 1, "c", 1))
---
>       if (str[1] == 'c')
...

The equivalent optimization is performed by GCC (but perhaps not by the
host compiler), but generating simpler/smaller code may encourage further
optimizations (such as use of a switch statement).

2022-05-24  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* genpreds.cc (write_lookup_constraint_1): Avoid generating a call
	to strncmp for strings of length one.