Sanitize the constant argument for rotr<mode>3

Sanitize the constant argument for rotr<mode>3

This was dormant for quite some time, but it started happening for me
on gcc.c-torture/compile/pr65153.c sometime after r276645 for -mabi=32 linux runs.

The pattern accepts any SMALL_OPERAND constant value while it asserts during the final
that the value is in the mode size range. I this case it happens that combine_and_move_insns
during ira makes a pattern with negative "shift count" which fails at final stage.

This simple fix just truncates the constant operand to mode size the same as shift patterns.

gcc/ChangeLog:

2019-11-13  Dragan Mladjenovic  <dmladjenovic@wavecomp.com>

	* config/mips/mips.md (rotr<mode>3): Sanitize the constant argument
	instead of asserting its value.

From-SVN: r278152