[Xtensa] Remove unnecessary MOVSP in epilogue. (LLVM-83)

[inetbowser]

This PR removes the use of an unnecessary MOVSP in the function epilogue (#19 ). Restoring the frame pointer when using Xtensa Windowed Regisers option is not needed because original stack pointer will get restored when returning (goes to register window of caller which contains original stack pointer).

For reference:
test.c:

int secretpin() {
    return 1337;
}

Compiling with xtensa-esp32-elf-gcc -c test.c:

00000000 <secretpin>:
   0:	004136        	entry	a1, 32
   3:	017d      	mov.n	a7, a1
   5:	39a522        	movi	a2, 0x539
   8:	f01d      	retw.n

Compiling with $LLVM_INSTALL_DIR/bin/clang --target="xtensa-freestanding" -mcpu=esp32 -c test.c:

00000000 <secretpin>:
   0:	004136        	entry	a1, 32
   3:	017d      	mov.n	a7, a1
   5:	39a522        	movi	a2, 0x539
   8:	001710        	movsp	a1, a7
   b:	f01d      	retw.n