fix(encodings): fix V extension encodings mismatch with assembly (#812)

Fixes #705. I believe the main problem @dbrash comment in
#705 (comment)
was that encodings and assembly were not matching, since I had fixed
assembly but not encodings. This PR should fix that.

---------

Signed-off-by: Afonso Oliveira <Afonso.Oliveira@synopsys.com>

Author: AFOliveira

arch/inst/V/vaadd.vx.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vaaddu.vx.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vadc.vim.yaml

@@ -13,7 +13,7 @@ encoding:
   variables:
     - name: vs2
       location: 24-20
-    - name: simm5
+    - name: imm
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vadc.vxm.yaml

@@ -13,7 +13,7 @@ encoding:
   variables:
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vadd.vi.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: simm5
+    - name: imm
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vadd.vx.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vand.vi.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: simm5
+    - name: imm
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vand.vx.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vasub.vx.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7

arch/inst/V/vasubu.vx.yaml

@@ -15,7 +15,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: rs1
+    - name: xs1
       location: 19-15
     - name: vd
       location: 11-7