x64: Remove conditional SseOpcode::uses_src1 (#5842)

This is a follow-up to comments in #5795 to remove some cruft in the x64
instruction model to ensure that the shape of an `Inst` reflects what's
going to happen in regalloc and encoding. This accessor was used to
handle `round*`, `pextr*`, and `pshufb` instructions. The `round*` ones
had already moved to the appropriate `XmmUnary*` variant and `pshufb`
was additionally moved over to that variant as well.

The `pextr*` instructions got a new `Inst` variant and additionally had
their constructors slightly modified to no longer require the type as
input. The encoding for these instructions now automatically handles the
various type-related operands through a new `SseOpcode::Pextrq` operand
to represent 64-bit movements.

cranelift/filetests/filetests/isa/x64/extractlane.clif

@@ -86,7 +86,7 @@ block0(v0: i64x2):
 ;   pushq   %rbp
 ;   movq    %rsp, %rbp
 ; block0:
-;   pextrd.w $1, %xmm0, %rax
+;   pextrq  $1, %xmm0, %rax
 ;   movq    %rbp, %rsp
 ;   popq    %rbp
 ;   ret

cranelift/filetests/filetests/isa/x64/simd-bitwise-compile.clif

@@ -753,8 +753,8 @@ block0(v0: i64x2):
 ;   pushq   %rbp
 ;   movq    %rsp, %rbp
 ; block0:
-;   pextrd.w $0, %xmm0, %rdx
-;   pextrd.w $1, %xmm0, %r9
+;   pextrq  $0, %xmm0, %rdx
+;   pextrq  $1, %xmm0, %r9
 ;   sarq    $36, %rdx, %rdx
 ;   sarq    $36, %r9, %r9
 ;   uninit  %xmm0
@@ -789,8 +789,8 @@ block0(v0: i64x2, v1: i32):
 ;   pushq   %rbp
 ;   movq    %rsp, %rbp
 ; block0:
-;   pextrd.w $0, %xmm0, %r8
-;   pextrd.w $1, %xmm0, %r10
+;   pextrq  $0, %xmm0, %r8
+;   pextrq  $1, %xmm0, %r10
 ;   movq    %rdi, %rcx
 ;   sarq    %cl, %r8, %r8
 ;   sarq    %cl, %r10, %r10