cmd/asm: VPERMQ's imm8 arg is an uint8

The imm8 argument consists of 4 2-bit indices, so it can take values up
to $255. However, the assembler was treating it as Yi8, which reads
"fits in int8". Add a Yu8 variant, to also keep backwards compatibility
with negative values possible with Yi8.

Fixes #24378.

Change-Id: I24ddb19c219b54d039a6c1bcdb903717d1c7c3b8
Reviewed-on: https://go-review.googlesource.com/100475
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/asm/internal/asm/testdata/amd64enc.s

@@ -8742,6 +8742,8 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	VPERMQ $7, (R11), Y11                   // c443fd001b07
 	VPERMQ $7, Y2, Y11                      // c463fd00da07
 	VPERMQ $7, Y11, Y11                     // c443fd00db07
+	VPERMQ $-40, Y8, Y8                     // c4407800c0d8
+	VPERMQ $216, Y8, Y8                     // c443fd00c0d8
 	VPEXTRB $7, X2, (BX)                    // c4e379141307
 	VPEXTRB $7, X11, (BX)                   // c46379141b07
 	VPEXTRB $7, X2, (R11)                   // c4c379141307

src/cmd/internal/obj/x86/asm6.go

@@ -952,6 +952,9 @@ var yvex_xi3 = []ytab{
 }
 
 var yvex_vpermpd = []ytab{
+	{Zvex_i_rm_r, 2, argList{Yu8, Yym, Yyr}},
+	// Allow int8 for backwards compatibility with negative values
+	// like $-1.
 	{Zvex_i_rm_r, 2, argList{Yi8, Yym, Yyr}},
 }