MSP430: Define extendqipsi2

The SXT instruction extends the sign of the low byte of the operand
through the entire PSImode register.
SXTX.A can be used to sign extend the low byte of a memory operand
through to the 19th bit. Bits 31:20 are cleared.

gcc/ChangeLog:

	* config/msp430/msp430.md: New "extendqipsi2" define_insn.

gcc/config/msp430/msp430.md

@@ -553,6 +553,15 @@
    SXT%X0\t%0"
 )
 
+(define_insn "extendqipsi2"
+  [(set (match_operand:PSI		   0 "msp430_general_dst_operand" "=r,m")
+	(sign_extend:PSI (match_operand:QI 1 "msp430_general_operand" "0,0")))]
+  ""
+  "@
+  SXT\t%0
+  SXTX.A\t%0"
+)
+
 ;; ------------------------
 ;; ZERO EXTEND INSTRUCTIONS
 ;; Byte-writes to registers clear bits 19:8